perf: optimize span count metric gathering (#65)

* perf: optimize span count metric gathering

instead of querying the database for the exact count of the span table
which is rather costly do a postgres stat lookup which is quite close
and extremely fast.

* enh: adapt query metrics in case spans table is partitioned

when spans table is partitioned the stats cannot be retrieved from the
parent table, with this change we retrieve the sum of all partitions
when avaiable

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Co-authored-by: Nicolás De los Santos <nicolas.delossantos@meteologica.com>

Author: aokiji