HDDS-14041. Add metrics to track Snapshot RocksDB space and SST File stats.

[sadanand48]

What changes were proposed in this pull request?

Add metrics to track Snapshot RocksDB space and SST File stats

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/HDDS-14041

How was this patch tested?

Tried out on docker

  "beans" : [ {
    "name" : "Hadoop:service=OzoneManager,name=OMSnapshotDirectoryMetrics",
    "modelerType" : "OMSnapshotDirectoryMetrics",
    "tag.Context" : "Snapshot Directory Metrics",
    "tag.Hostname" : "b8f9c29c90ba",
    "DbSnapshotsDirSize" : 1162516,
    "TotalSstFilesCount" : 36,
    "NumSnapshots" : 3
  } ]
}

[jojochuang]

@sadanand48 also I updated the jira release note. PTAL