backend: more metrics for bboltdb transaction

[xiang90]

@gyuho @jpbetz

We observed high latency in the commit call. Adding more metrics can help to understand which part of bbolt commit is the root cause.

[jpbetz]

There was a question in the last etcd community meeting of if we should keep the "etcd_debugging" but since we don't have a final decision on that, I think it makes sense to continue to use it.

The histograms look good to me. Thanks!

[gyuho]

lgtm. thanks @xiang90 !

[gyuho]

Added change log and missing prometheus register calls. Merging in greens.

[gyuho]

Also replaced metrics naming to _ /cc @xiang90

panic: descriptor Desc{fqName: "etcd_debugging_disk_backend_commit.rebalance_duration_seconds", help: "The latency distributions of commit.rebalance called by bboltdb backend.", constLabels: {}, variableLabels: []} is invalid: "etcd_debugging_disk_backend_commit.rebalance_duration_seconds" is not a valid metric name

[xiang90]

@gyuho Thank you!

[jpbetz]

cc @wenjiaswe