[Snapshot Interop] Add per shard metric for number of locks on remote store metadata

[harishbhakuni]

Is your feature request related to a problem? Please describe.
With Snapshot Interoperability w/ remote store feature, we are adding a lock manager in remote store which will keep lock files for remote store metadata. To track how many lock files are there for a given shard, we should add per shard metric for number of locks on remote store metadata.

Describe the solution you'd like
Add per shard metric for number of locks on remote store metadata. we can later use it for dangling locks monitor as well: #7501.

[sachinpkale]

@harishbhakuni Please add more info on how this metric would help user.