storage: iterator stats should consider block count

We currently have iterator stats reported in trace bundles that look as follows:

```
event:scan stats: stepped 0 times (0 internal); seeked 1 times (1 internal); block-bytes: (total 895 KiB, cached 883 KiB, duration 1.533227ms); points: (count 1, key-bytes 65 B, value-bytes 88 B, tombstoned: 0) ranges: (count 0), (contained-points 0, skipped-points 0) evaluated requests: 1 gets, 0 scans, 0 reverse scans
```

It would be useful to include a count of total block "touches" and block misses.

This will require a change down in Pebble to accumulate the stats. And then a second change to wire up the stats in the protos in Cockroach to allow the trace data to be propagated.

More context [here](https://cockroachlabs.slack.com/archives/CAC6K3SLU/p1717559256220939) (internal).

Jira issue: CRDB-39251