add clustering_max_lag to /metrics #4422

Job	Run time
Format check	18s
Lint javascript	15s
Lint check	1m 6s
Compile LavinMQ	1m 12s
Spec FreeBSD	2m 21s
Lint OpenAPI	18s
Spec OS X	2m 38s
Spec	2m 27s
Filters for frontend specs	4s
Bunny client test	2m 36s
RabbitMQ HTTP API Go client test	57s
RabbitMQ java client test	5m 5s
Test in release mode	15s
RabbitMQ HTTP API Ruby client test	55s
Frontend specs	0s
	20m 27s

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