Collect statistics for distinct commands on mongos. This feature should be feature flag guarded.

Distinct has a different entrypoint on mongos, and lives at cluster_distinct_cmd.cpp. We should follow the same steps as standalone (register the request, collect metrics and write to the query stats store). To help collect the query stats, there are specific mongos helper functions that are used by cluster_aggregate and cluster_find. The key difference between query stats for find/agg and query stats for distinct is that distinct doesn't open a cursor and returns a RemoteCommandResponse.

This ticket should not add any data bearing node metrics, since that will be handled by SERVER-90651.