mma: overload dur metrics succes + failure != total

The overloaded long duration success and failure metrics do not sum to the total count of overloaded stores. This is because ranges for which this store does not hold the leaseholder replica are skipped entirely and not counted toward either metric. In addition, if all ranges are skipped (then neither a success nor a failure), this does not count towards either metric.

Epic: CRDB-56265

Start by searching the Go code for the overloaded long-duration success, failure, and total metrics, then trace how ranges are counted when this store lacks the leaseholder replica. Done means skipped ranges, including the all-skipped case, are represented so success plus failure matches the overloaded-store total.

Written by the indexing model from the issue text.