Counter metric for number of queries in slow query log.

**Is your feature request related to a problem? Please describe.**
There is a need to surface the number of queries in the slow query log to understand where the load is coming from on a heavily used system.

**Describe the solution you'd like**
A counter metric to expose the number of queries in the slow query log exposed as an endpoint. It would be helpful to include a counter for the tables involved and the query `statement_id` as well.

Jira issue: CRDB-24670

Epic CRDB-32139

No files or tests are identified. Start by locating the slow query log and the metrics endpoint, then determine how query counts, involved tables, and statement_id values should be represented. Done means the endpoint exposes the requested counter metrics and has coverage demonstrating their reported values.

Written by the indexing model from the issue text.