storage: thread metrics entries for BackingTableSize/Count

To improve observability around virtual sstables, we need to thread through the `BackingTableSize` and `BackingTableCount` metrics from Pebble's `Metrics.Table` struct through to `eventpb.StoreStats` (as returned by `AsStoreStatsEvent()` in pkg/storage/engine.go). We also need to add a new metrics.Gauge in pkg/kv/kvserver's StoreMetrics for these two metrics, and update them in StoreMetrics' `updateEngineMetrics`.

As virtual sstable sizes/counts will often be compared to total sstable table/counts (as the latter reflects the virtualized views of sstables), we will also need to confirm if there are easy-to-grok metrics for total sstable sizes/counts. Currently it looks like StoreMetrics.RdbLevelSize is a set of 7 size metrics, and it would be a good idea to add a `TotalLevelSize` that adds them all up.

Jira issue: CRDB-35642