db: space amp due to virtual tables not accounted in Pebble metrics

We compute an estimated size for each virtual sst based on the blocks it references in the backing sst. And we correctly use the backing sst size when computing the aggregate size of the DB (for all the metrics that are used in the calculation of `Metrics.DiskSpaceUsage()`).
But we don't account for the space amp e.g. if backing sst0 has size 100 and is referenced by sst1 and sst2 that have size 10 and 20 respectively, then 100-(10+20)=70 is garbage. When we see higher than expected size of a DB, being able to put an exact number to such space amp is useful. Came up in the context of https://github.com/cockroachlabs/support/issues/3264

Jira issue: PEBBLE-410