sql: periodically flush sqlstats

[jseldess]

Jesse Seldess (jseldess) commented:

PR: cockroachdb/cockroach#67090

From release notes:

SQL stats now can be persisted into system.statement_statistics and system.transaction_statistics tables by enabling the sql.stats.flush.enable [cluster setting](../v21.2/cluster-settings.html). The interval of persistence is determined by the new sql.stats.flush.interval [cluster setting](../v21.2/cluster-settings.html), which defaults to 1 hour. #67090#67090

Discussed in other forums, we’re not yet documenting cluster settings (flush interval) for persistence statistics. There are no guardrails for this cluster setting or best practices established. The 1-hour should be sufficient for majority of clusters and if not, we’d like to hear the feedback.

Jira Issue: DOC-1384