GSI Collections: Support collection aware stats

ns-server periodically calls the /stats endpoint of the indexer. The output is a json of all the stats. The indexes are prefixed with “bucket:” to make each index entry unique. The new naming scheme needs to be agreed upon between ns-server and GSI team.

Lot of the current stats are based on seqnums. E.g when an index gets built, it will list the number of “Mutations Pending” based on seqnums of vbuckets. KV will support collection aware GET_ALL_VB_SEQNO and indexer needs to utilize this to make sure all the stats are meaningful.

Evaluate prometheus endpoint for stats.

Identify per collection stats requirement.

Remove unused stats.