Metrics: Need to add metric to shard capacity

[anatoly-bogatyrev]

Metrics: Need to add metric to shard capacity.

The goal is to be able to obtain information about shard size and how much is filled with data.
It is necessary to provide a separate accounting for blobstore, metadata, writecache, blobovnitzas?

This issue requires a separate discussion of how and in what way we want to take into account shards capacity.