storage: expand time-series metrics for visibility into deletions

[jbowens]

It can be difficult to tell if data that was logically dropped by range or point deletions has been physically dropped. Some ideas for new time-series metrics that can help here:

• Total count of range deletions across the LSM.
• Total count of point deletions across the LSM.
• Total sum of the RangeDeletionBytesEstimate.
• LSM Snapshot metrics (eg, db: improve visibility into snapshots pebble#1204)

Jira issue: CRDB-9830

Epic CRDB-34247

[jbowens]

We could call the remaining items superseded by #102604. I'm not sure timeseries for the remaining two is much more valuable than the ability to aggregate those stats through the new crdb_internal func.