Add hook for statsd integration

[5antelope]

Signed-off-by: crowu y.wu4515@gmail.com

Description

This PR adds a hook interface for different backend to register timing and histogram metrics.

The main motivation comes from the integration with statsd:
the statsd API for timing / histogram takes a name and value, and it will aggregate the percentile metrics for us. This makes it really hard to parse the histogram data from vitess as latency in statsd.
We currently just represent them as a plain gauges with bucket suffix - this is not a very helpful metrics to understand the latency performance.

This PR adds a statsdHook, different stats backend can choose to add different hooks for timing and histogram, and when we add timing / histogram, Vitess will also try to invoke the hook function as well.

One caveat: this setup only applies to stats that published after the register functions in servenv.OnRun being called. For metrics that published before vitess server starts, we will just ignore them since we haven't register the hook yet

Although we are plan to integrate with OpenTelemetry , I feel there is still value to add this support to unblock any existing statsd use case.

Related Issue(s)

• Add a hook func to stats.Timings  #7298

Checklist

• Should this PR be backported?
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

NA

Impacted Areas in Vitess

Components that this PR will affect:

• Query Serving - mostly for stats reporting
• VReplication
• Cluster Management
• Build/CI
• VTAdmin

[5antelope]

I don't think the failing test is caused by the change... is there a way to manually trigger the CI builds again? 🤔

[5antelope]

Looks like all tests are passed after reran them!
Thanks @deepthi for helping :-)

[sougou]

Sorry about the delayed review. This looks good. Some tests didn't complete, and no way to restart. But they don't look related. I'll just force-merge.