Measure GIN index pending-list sizes

Re-introduce the pending-list size metric, as one of the follow-ups to gitlab-org/gitlab#376699 (comment 1125432039)

This metric had previously been created but was disabled due its query being incompatible with partitioned indexes.

Before merging this, we first need to merge the fix for the query in the function called by this metric: gitlab-com/gl-infra/production!161 (merged)