diff --git a/src/Metrics/MetricsAggregator.php b/src/Metrics/MetricsAggregator.php
index edd448772..5014c97fe 100644
--- a/src/Metrics/MetricsAggregator.php
+++ b/src/Metrics/MetricsAggregator.php
@@ -64,7 +64,7 @@ public function add(
             $type .
             $key .
             $unit .
-            implode('', $tags) .
+            serialize($tags) .
             $bucketTimestamp
         );
 
diff --git a/src/Serializer/EnvelopItems/TransactionItem.php b/src/Serializer/EnvelopItems/TransactionItem.php
index 5296299be..acb0272c4 100644
--- a/src/Serializer/EnvelopItems/TransactionItem.php
+++ b/src/Serializer/EnvelopItems/TransactionItem.php
@@ -177,7 +177,16 @@ protected static function serializeSpan(Span $span): array
         }
 
         if (!empty($span->getMetricsSummary())) {
-            $result['metrics_summary'] = $span->getMetricsSummary();
+            $formattedSummary = [];
+            $summary = $span->getMetricsSummary();
+
+            foreach ($summary as $mri => $metrics) {
+                foreach ($metrics as $metric) {
+                    $formattedSummary[$mri][] = $metric;
+                }
+            }
+
+            $result['_metrics_summary'] = $formattedSummary;
         }
 
         return $result;
diff --git a/src/Tracing/Span.php b/src/Tracing/Span.php
index f743d0aa9..c2c0f79ab 100644
--- a/src/Tracing/Span.php
+++ b/src/Tracing/Span.php
@@ -89,7 +89,7 @@ class Span
     protected $transaction;
 
     /**
-     * @var array<string, MetricsSummary>
+     * @var array<string, array<string, MetricsSummary>>
      */
     protected $metricsSummary = [];
 
@@ -492,7 +492,7 @@ public function detachSpanRecorder()
     }
 
     /**
-     * @return array<string, mixed>
+     * @return array<string, array<string, MetricsSummary>>
      */
     public function getMetricsSummary(): array
     {
@@ -511,17 +511,20 @@ public function setMetricsSummary(
         array $tags
     ): void {
         $mri = sprintf('%s:%s@%s', $type, $key, (string) $unit);
-        $bucketKey = $mri . implode('', $tags);
+        $bucketKey = $mri . serialize($tags);
 
-        if (\array_key_exists($bucketKey, $this->metricsSummary)) {
+        if (
+            isset($this->metricsSummary[$mri])
+            && \array_key_exists($bucketKey, $this->metricsSummary[$mri])
+        ) {
             if ($type === SetType::TYPE) {
                 $value = 1.0;
             } else {
                 $value = (float) $value;
             }
 
-            $summary = $this->metricsSummary[$bucketKey];
-            $this->metricsSummary[$bucketKey] = [
+            $summary = $this->metricsSummary[$mri][$bucketKey];
+            $this->metricsSummary[$mri][$bucketKey] = [
                 'min' => min($summary['min'], $value),
                 'max' => max($summary['max'], $value),
                 'sum' => $summary['sum'] + $value,
@@ -535,7 +538,7 @@ public function setMetricsSummary(
                 $value = (float) $value;
             }
 
-            $this->metricsSummary[$bucketKey] = [
+            $this->metricsSummary[$mri][$bucketKey] = [
                 'min' => $value,
                 'max' => $value,
                 'sum' => $value,
diff --git a/tests/Metrics/MetricsTest.php b/tests/Metrics/MetricsTest.php
index 354030f1f..8dc7388a8 100644
--- a/tests/Metrics/MetricsTest.php
+++ b/tests/Metrics/MetricsTest.php
@@ -40,7 +40,7 @@ public function testIncrement(): void
         $client->expects($this->once())
             ->method('captureEvent')
             ->with($this->callback(static function (Event $event) use ($self): bool {
-                $metric = $event->getMetrics()['92ed00fdaf9543ff4cace691f8a5166b'];
+                $metric = $event->getMetrics()['2794a118fd879e10a3a97836df803872'];
 
                 $self->assertSame(CounterType::TYPE, $metric->getType());
                 $self->assertSame('foo', $metric->getKey());
@@ -102,7 +102,7 @@ public function testDistribution(): void
         $client->expects($this->once())
             ->method('captureEvent')
             ->with($this->callback(static function (Event $event) use ($self): bool {
-                $metric = $event->getMetrics()['8a817dcdb12cfffc1fa8b459ad0c9d56'];
+                $metric = $event->getMetrics()['edb74f95b4572e82dc4600cfeea76181'];
 
                 $self->assertSame(DistributionType::TYPE, $metric->getType());
                 $self->assertSame('foo', $metric->getKey());
@@ -162,30 +162,30 @@ public function testTiming(): void
         $self = $this;
 
         $client->expects($this->once())
-               ->method('captureEvent')
-               ->with($this->callback(static function (Event $event) use ($self): bool {
-                   $metric = $event->getMetrics()['8a817dcdb12cfffc1fa8b459ad0c9d56'];
-
-                   $self->assertSame(DistributionType::TYPE, $metric->getType());
-                   $self->assertSame('foo', $metric->getKey());
-                   $self->assertSame([1.0, 2.0], $metric->serialize());
-                   $self->assertSame(MetricsUnit::second(), $metric->getUnit());
-                   $self->assertSame(
-                       [
-                           'environment' => 'development',
-                           'foo' => 'bar',
-                           'release' => '1.0.0',
-                       ],
-                       $metric->getTags()
-                   );
-                   $self->assertSame(1699412953, $metric->getTimestamp());
-
-                   $codeLocation = $metric->getCodeLocation();
-
-                   $self->assertSame('Sentry\Metrics\Metrics::timing', $codeLocation->getFunctionName());
-
-                   return true;
-               }));
+            ->method('captureEvent')
+            ->with($this->callback(static function (Event $event) use ($self): bool {
+                $metric = $event->getMetrics()['edb74f95b4572e82dc4600cfeea76181'];
+
+                $self->assertSame(DistributionType::TYPE, $metric->getType());
+                $self->assertSame('foo', $metric->getKey());
+                $self->assertSame([1.0, 2.0], $metric->serialize());
+                $self->assertSame(MetricsUnit::second(), $metric->getUnit());
+                $self->assertSame(
+                    [
+                        'environment' => 'development',
+                        'foo' => 'bar',
+                        'release' => '1.0.0',
+                    ],
+                    $metric->getTags()
+                );
+                $self->assertSame(1699412953, $metric->getTimestamp());
+
+                $codeLocation = $metric->getCodeLocation();
+
+                $self->assertSame('Sentry\Metrics\Metrics::timing', $codeLocation->getFunctionName());
+
+                return true;
+            }));
 
         $hub = new Hub($client);
         SentrySdk::setCurrentHub($hub);
@@ -238,7 +238,7 @@ public function testGauge(): void
         $client->expects($this->once())
             ->method('captureEvent')
             ->with($this->callback(static function (Event $event) use ($self): bool {
-                $metric = $event->getMetrics()['d2a09273b9c61b66a0e6ee79c1babfed'];
+                $metric = $event->getMetrics()['f39c23c73897d4f006fd617f76664571'];
 
                 $self->assertSame(GaugeType::TYPE, $metric->getType());
                 $self->assertSame('foo', $metric->getKey());
@@ -306,7 +306,7 @@ public function testSet(): void
         $client->expects($this->once())
             ->method('captureEvent')
             ->with($this->callback(static function (Event $event) use ($self): bool {
-                $metric = $event->getMetrics()['c900a5750d0bc79016c29a7f0bdcd937'];
+                $metric = $event->getMetrics()['868b190d923bbd619570328d7ba3e4cd'];
 
                 $self->assertSame(SetType::TYPE, $metric->getType());
                 $self->assertSame('foo', $metric->getKey());
diff --git a/tests/Serializer/PayloadSerializerTest.php b/tests/Serializer/PayloadSerializerTest.php
index 19e00acc8..2b7f775ba 100644
--- a/tests/Serializer/PayloadSerializerTest.php
+++ b/tests/Serializer/PayloadSerializerTest.php
@@ -319,7 +319,6 @@ public static function serializeAsEnvelopeDataProvider(): iterable
 {"timestamp":1597790835,"platform":"php","sdk":{"name":"sentry.php","version":"$sdkVersion"},"spans":[],"transaction_info":{"source":"custom"}}
 TEXT
             ,
-            false,
         ];
 
         $event = Event::createEvent(new EventId('fc9442f5aef34234bb22b9a615e30ccd'));
@@ -411,7 +410,6 @@ public static function serializeAsEnvelopeDataProvider(): iterable
 {"check_in_id":"$checkinId","monitor_slug":"my-monitor","status":"ok","duration":10,"release":"1.0.0","environment":"dev","monitor_config":{"schedule":{"type":"crontab","value":"0 0 * * *","unit":""},"checkin_margin":10,"max_runtime":12,"timezone":"Europe\/Amsterdam","failure_issue_threshold":5,"recovery_threshold":10},"contexts":{"trace":{"trace_id":"21160e9b836d479f81611368b2aa3d2c","span_id":"5dd538dc297544cc"}}}
 TEXT
             ,
-            false,
         ];
 
         $counter = new CounterType('counter', 1.0, MetricsUnit::second(), ['foo' => 'bar', 'baz' => 'qux'], 1597790835);
@@ -441,7 +439,46 @@ public static function serializeAsEnvelopeDataProvider(): iterable
 no_tags@second:1|c|T1597790835
 TEXT
             ,
-            false,
+        ];
+
+        $span = new Span();
+        $span->setSpanId(new SpanId('5dd538dc297544cc'));
+        $span->setTraceId(new TraceId('21160e9b836d479f81611368b2aa3d2c'));
+        $span->setMetricsSummary(
+            CounterType::TYPE,
+            'counter',
+            10,
+            MetricsUnit::custom('star'),
+            [
+                'repository' => 'client',
+            ]
+        );
+        $span->setMetricsSummary(
+            CounterType::TYPE,
+            'counter',
+            50,
+            MetricsUnit::custom('star'),
+            [
+                'repository' => 'client',
+            ]
+        );
+
+        $event = Event::createTransaction(new EventId('fc9442f5aef34234bb22b9a615e30ccd'));
+        $event->setSpans([$span]);
+        $event->setTransaction('GET /');
+        $event->setContext('trace', [
+            'trace_id' => '21160e9b836d479f81611368b2aa3d2c',
+            'span_id' => '5dd538dc297544cc',
+        ]);
+
+        yield [
+            $event,
+            <<<TEXT
+{"event_id":"fc9442f5aef34234bb22b9a615e30ccd","sent_at":"2020-08-18T22:47:15Z","dsn":"http:\/\/public@example.com\/sentry\/1","sdk":{"name":"sentry.php","version":"4.4.0"}}
+{"type":"transaction","content_type":"application\/json"}
+{"timestamp":1597790835,"platform":"php","sdk":{"name":"sentry.php","version":"4.4.0"},"transaction":"GET \/","contexts":{"trace":{"trace_id":"21160e9b836d479f81611368b2aa3d2c","span_id":"5dd538dc297544cc"}},"spans":[{"span_id":"5dd538dc297544cc","trace_id":"21160e9b836d479f81611368b2aa3d2c","start_timestamp":1597790835,"_metrics_summary":{"c:counter@star":[{"min":10,"max":50,"sum":60,"count":2,"tags":{"repository":"client"}}]}}]}
+TEXT
+            ,
         ];
     }
 }
diff --git a/tests/Tracing/SpanTest.php b/tests/Tracing/SpanTest.php
index 406118333..0f851a092 100644
--- a/tests/Tracing/SpanTest.php
+++ b/tests/Tracing/SpanTest.php
@@ -5,6 +5,11 @@
 namespace Sentry\Tests\Tracing;
 
 use PHPUnit\Framework\TestCase;
+use Sentry\Metrics\MetricsUnit;
+use Sentry\Metrics\Types\CounterType;
+use Sentry\Metrics\Types\DistributionType;
+use Sentry\Metrics\Types\GaugeType;
+use Sentry\Metrics\Types\SetType;
 use Sentry\Tracing\Span;
 use Sentry\Tracing\SpanContext;
 use Sentry\Tracing\SpanId;
@@ -140,4 +145,131 @@ public static function toBaggageDataProvider(): iterable
             'sentry-public_key=public,sentry-trace_id=566e3688a61d4bc888951642d6f14a19,sentry-sample_rate=1',
         ];
     }
+
+    public function testMetricsSummary(): void
+    {
+        $span = new Span();
+        $span->setMetricsSummary(
+            CounterType::TYPE,
+            'counter',
+            10,
+            MetricsUnit::custom('star'),
+            [
+                'repository' => 'client',
+            ]
+        );
+        $span->setMetricsSummary(
+            CounterType::TYPE,
+            'counter',
+            50,
+            MetricsUnit::custom('star'),
+            [
+                'repository' => 'client',
+            ]
+        );
+        $span->setMetricsSummary(
+            CounterType::TYPE,
+            'counter',
+            10,
+            MetricsUnit::custom('star'),
+            [
+                'repository' => 'server',
+            ]
+        );
+
+        $span->setMetricsSummary(
+            DistributionType::TYPE,
+            'distribution',
+            10.2,
+            MetricsUnit::millisecond(),
+            []
+        );
+        $span->setMetricsSummary(
+            DistributionType::TYPE,
+            'distribution',
+            5.7,
+            MetricsUnit::millisecond(),
+            []
+        );
+
+        $span->setMetricsSummary(
+            GaugeType::TYPE,
+            'gauge',
+            10,
+            MetricsUnit::none(),
+            []
+        );
+        $span->setMetricsSummary(
+            GaugeType::TYPE,
+            'gauge',
+            20,
+            MetricsUnit::none(),
+            []
+        );
+
+        $span->setMetricsSummary(
+            SetType::TYPE,
+            'set',
+            'jane@doe@example.com',
+            MetricsUnit::custom('user'),
+            []
+        );
+        $span->setMetricsSummary(
+            SetType::TYPE,
+            'set',
+            'jon@doe@example.com',
+            MetricsUnit::custom('user'),
+            []
+        );
+
+        $this->assertSame([
+            'c:counter@star' => [
+                'c:counter@stara:1:{s:10:"repository";s:6:"client";}' => [
+                    'min' => 10.0,
+                    'max' => 50.0,
+                    'sum' => 60.0,
+                    'count' => 2,
+                    'tags' => [
+                        'repository' => 'client',
+                    ],
+                ],
+                'c:counter@stara:1:{s:10:"repository";s:6:"server";}' => [
+                    'min' => 10.0,
+                    'max' => 10.0,
+                    'sum' => 10.0,
+                    'count' => 1,
+                    'tags' => [
+                        'repository' => 'server',
+                    ],
+                ],
+            ],
+            'd:distribution@millisecond' => [
+                'd:distribution@milliseconda:0:{}' => [
+                    'min' => 5.7,
+                    'max' => 10.2,
+                    'sum' => 15.899999999999999,
+                    'count' => 2,
+                    'tags' => [],
+                ],
+            ],
+            'g:gauge@none' => [
+                'g:gauge@nonea:0:{}' => [
+                    'min' => 10.0,
+                    'max' => 20.0,
+                    'sum' => 30.0,
+                    'count' => 2,
+                    'tags' => [],
+                ],
+            ],
+            's:set@user' => [
+                's:set@usera:0:{}' => [
+                    'min' => 0.0,
+                    'max' => 1.0,
+                    'sum' => 1.0,
+                    'count' => 2,
+                    'tags' => [],
+                ],
+            ],
+        ], $span->getMetricsSummary());
+    }
 }