[ring_hash] allow use without xDS, and allow setting endpoint hash ke…

…y in EDS (grpc#38312)

As per gRFC A76 (grpc/proposal#412).

Closes grpc#38312

COPYBARA_INTEGRATE_REVIEW=grpc#38312 from markdroth:ring_hash_changes ae691bb
PiperOrigin-RevId: 708054036

BUILD

@@ -605,7 +605,6 @@ GRPC_XDS_TARGETS = [
     "//src/core:grpc_lb_policy_xds_cluster_manager",
     "//src/core:grpc_lb_policy_xds_override_host",
     "//src/core:grpc_lb_policy_xds_wrr_locality",
-    "//src/core:grpc_lb_policy_ring_hash",
     "//src/core:grpc_resolver_xds",
     "//src/core:grpc_resolver_c2p",
     "//src/core:grpc_xds_server_config_fetcher",
@@ -838,6 +837,7 @@ grpc_cc_library(
         "//src/core:grpc_lb_policy_outlier_detection",
         "//src/core:grpc_lb_policy_pick_first",
         "//src/core:grpc_lb_policy_priority",
+        "//src/core:grpc_lb_policy_ring_hash",
         "//src/core:grpc_lb_policy_round_robin",
         "//src/core:grpc_lb_policy_weighted_round_robin",
         "//src/core:grpc_lb_policy_weighted_target",

src/core/BUILD

@@ -6011,6 +6011,7 @@ grpc_cc_library(
         "grpc_fake_credentials",
         "grpc_fault_injection_filter",
         "grpc_lb_policy_pick_first",
+        "grpc_lb_policy_ring_hash",
         "grpc_lb_xds_channel_args",
         "grpc_matchers",
         "grpc_outlier_detection_header",
@@ -6667,6 +6668,7 @@ grpc_cc_library(
         "absl/container:inlined_vector",
         "absl/log:check",
         "absl/log:log",
+        "absl/random",
         "absl/status",
         "absl/status:statusor",
         "absl/strings",
@@ -6679,6 +6681,7 @@ grpc_cc_library(
         "closure",
         "connectivity_state",
         "delegating_helper",
+        "env",
         "error",
         "grpc_lb_policy_pick_first",
         "grpc_service_config",
@@ -6690,6 +6693,7 @@ grpc_cc_library(
         "lb_policy_registry",
         "pollset_set",
         "ref_counted",
+        "ref_counted_string",
         "resolved_address",
         "unique_type_name",
         "validation_errors",

src/core/load_balancing/lb_policy.h

@@ -152,14 +152,6 @@ class LoadBalancingPolicy : public InternallyRefCounted<LoadBalancingPolicy> {
     /// The LB policy may use the existing metadata to influence its routing
     /// decision, and it may add new metadata elements to be sent with the
     /// call to the chosen backend.
-    // TODO(roth): Before making the LB policy API public, consider
-    // whether this is the right way to expose metadata to the picker.
-    // This approach means that if a pick modifies metadata but then we
-    // discard the pick because the subchannel is not connected, the
-    // metadata change will still have been made.  Maybe we actually
-    // want to somehow provide metadata changes in PickResult::Complete
-    // instead?  Or maybe we use a CallTracer that can add metadata when
-    // the call actually starts on the subchannel?
     MetadataInterface* initial_metadata;
     /// An interface for accessing call state.  Can be used to allocate
     /// memory associated with the call in an efficient way.