mgr/telemetry: modify stats_per_pool

mgr/telemetry: modify stats_per_pool

There is a much easier way to collect stats_per_pool than the current implementation. Fetching 'pg_dump' from the mgr module already provides a field called "pool_stats" that is the same as aggregated pg stats, which was the implementation up until this commit.

All in all, this solution should provide the information we want, with a much cleaner implementation.

Signed-off-by: Laura Flores <lflores@redhat.com>
Backport Message: In the case that this commit is backported, it is important to note that the commits in PR #42569 should be backported first, as the implementation of "get_stat_sum_per_pool()" in #42569 precedes the removal of it here.