persist: keep per-shard series that match an enable regex - #39601

Problem

When persist_shard_metrics leaves out the per-shard families (the
summary and none modes), we lose the per-shard drill-down
that we might need.

E.g., mz_persist_shard_update_count{name=$source}.

The aggregates show how bad things are and the heaviest hitter but not
all the metrics for one shard. However, turning per-shard series back on
for the whole environment would bring the cardinality problem right back.

Solution

Add persist_per_shard_metrics_enable_regex, if the regex isn't empty
it keeps the series whose shard or name label matches, and drops the rest.

Testing

New tests check that matching a shard id brings back exactly that
shard's series in all 28 families, identical to the ones in the both
mode, and that matching by name and by an alternation keeps exactly
the matched shards. They also check that an empty regex keeps none, and
that an unanchored pattern over-matches as documented: it keeps every
name it is a substring of, and a fragment of one shard's id also keeps
shards whose name contains it

Closes: PER-103