Provide better observability for follower reads

This tracks providing better observability for statements and transactions that are using follower reads. More specifically, CockroachDB should provide a way for users to understand the distribution of backdated values/timestamps. This observability will help with troubleshooting scenarios such as contention and understanding potential causes of outliers. We should consider touchpoints such as Insights and SQL Activity pages.

Related issue(s):

- https://github.com/cockroachdb/cockroach/issues/94751
- https://github.com/cockroachdb/cockroach/issues/63983

Jira issue: CRDB-23490

Read related issues 94751 and 63983 first, then examine the Insights and SQL Activity pages named in this issue. Define how statements and transactions using follower reads should expose the distribution of backdated values or timestamps, with completion shown by usable observability for troubleshooting contention and outliers.

Written by the indexing model from the issue text.