Add more labels to VReplicationSecondsBehindMaster

[zmagg]

Description

Adding more labels to VReplicationSecondsBehindMaster.

@sougou I know we talked about how you're refactoring tabletmanager, but after taking a closer look at this metric, it was pretty simple to emit the extra labels now. 😁 Let me know if this gets in the way of the refactor somehow.

Testing

Tested in a dev env, and the emitted metrics now look like:

(debug/vars version)

"VReplicationSecondsBehindMaster": {"test_keyspace.-80.test_workflow.3": 0, "test_keyspace.80-.test_workflow.4": 0},

(/metrics version)

vttablet_v_replication_seconds_behind_master{counts="3",source_keyspace="test_keyspace",source_shard="-80",workflow="test_workflow"} 0

[sougou]

I misunderstood what you were trying to tell me (confused with vttablet labels). This is a great idea, and shouldn't interfere with labels. LMK if you want me to merge this as is or if you plan to do more work.

[sougou]

One suggestion: Since id is the most specific of the labels, I'd go with workflow.keyspace.shard.id.

[zmagg]

Oh good idea, updated.

Fine to merge as is now, I just wanted to check to make sure the labels were emitted as expected in dev where we have a stream running, and they are! Updated the description accordingly.