queryExecutionHook OnQueryCompleted

[erikhalperin]

Description

This PR adds a QueryExecutionHook interface which runs ProcessQueryCompleted when a query is completed (by Execute) in the vttablet. By default, this will do nothing, but it will allow users to implement QueryExecutionHook and do whatever they want inside (which in HubSpot's case would be to add prometheus-backed p99 and median vttablet latencies)

See the attached issue for the background, but I hope this can be a non-intrusive way to accomplish adding p99 latency metrics.

Adding tests & documentation is in progress

Related Issue(s)

Fixes #18597

Checklist

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Deployment Notes

[vitess-bot]

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