sql: index usage metrics for unused indexes (reads)

Feature brief: [Index usage statistics](https://docs.google.com/document/d/1M3Rj3YJTsY8z45BRYgy3H1qs9mZLfm1vXEKl5MdIO2s/edit?usp=sharing)
Summary: Capture metrics for when indexes are used for reads

This issue captures V1 index usage metrics (reads) for the unused index user story. 

Epic: CRDB-6570