kvserver: use count of encountered MVCC tombstones as GC signal

[tbg]

Is your feature request related to a problem? Please describe.

See #17229 (comment). In this issue, @a-robinson ended up with a range that wasn't eligible for GC even though most of the shadowed data was GC'able, because the GC'able data represented only a small portion of the range. However, this small portion of the range was hot and so performance suffered.

Describe the solution you'd like

We could capture stats the rate at which shadowed versions are skipped and use this to influence the aggressiveness of GC. In particular, the DeadFraction¹ should not lower the score when the shadowed data is read at a high rate.

Jira issue: CRDB-25331

Footnotes

1. https://github.com/cockroachdb/cockroach/blob/736a67e0d36cc545bf74d65db069ee895ff9bea0/pkg/kv/kvserver/mvcc_gc_queue.go#L457 ↩