add message rates to /exchanges #5022

Job	Run time
Format check	11s
Lint check	1m 2s
Lint OpenAPI	15s
Lint javascript	18s
Compile LavinMQ	1m 1s
Filters for frontend specs	3s
Spec	2m 3s
Spec FreeBSD	2m 41s
Spec OS X	2m 58s
RabbitMQ java client test	5m 12s
Bunny client test	2m 32s
RabbitMQ HTTP API Go client test	1m 2s
Test in release mode	19s
RabbitMQ HTTP API Ruby client test	51s
Frontend specs	3m 58s
	24m 26s

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