Add more vstream metrics for vstream manager

[twthorn]

Description

Add more metrics for vstreams on vtgates.

Inspired partially by the tablet vstream metrics. Let me know if you think I should include any others. Once we are decided on the metrics list, I will put out docs PR.

Related Issue(s)

• Fixes: Feature Request: VTGates should report more metrics on vstreams #17859

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation: Document new metrics for vtgate vstream website#1948

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.47%. Comparing base (81ce29c) to head (239367b).
Report is 20 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #17858    +/-   ##
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  Coverage   67.46%   67.47%            
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  Files        1593     1594     +1     
  Lines      258885   259089   +204     
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+ Hits       174652   174814   +162     
- Misses      84233    84275    +42     

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[mattlord]

Thank you for working on this, @twthorn ! ❤️ I think that we should work out what we want these metrics to represent, and then we can refine from there. Hopefully all of my comments make sense?

[mattlord]

I had assumed that this was a count of vtgate vstreams rather than tablet streams. Is this really supposed to be a count of tablet streams (one per shard, per vtgate vstream)?

[twthorn]

I believe the most useful for observability is to have per shard per vtgate stream. For example, with the lag metric, if the vstream has multiple shards, we cannot accurately report lag (do we do the max, min, avg, etc.).

I believe the same is true

1. errors - it saves operator the time to find in a log which stream has error
2. active streams - if the vtgate is operating slow the vstream count is not enough granularity if those vstreams have hundreds of shards, we want to know how many shards this vtgate is handling
3. events streamed - help operator detect which shard has disproportionately high load

I agree per tablet is maybe too granular, but per-shard would be very useful. I am going to update PR based on this

[mattlord]

OK, makes sense. Thanks!

[mattlord]

LGTM! I only had some minor comments and suggestions. Please let me know what you think.

Thanks again, @twthorn !

[mattlord]

I don't see any value in adding 0. Am I missing something?

[twthorn]

This is to initialize the counter to zero for this keyspace/shard/tablet type. This is a best practice. Metrics are hard to work with if they only sometimes exist. This is also seen in the unit test for metrics that we can't assert that there were zero errors for a vstream that worked fine (the metric is simply missing)

[twthorn]

I changed to Reset to hopefully make it more clear, as Add zero does appear to be not useful

[mattlord]

Is Reset what you really want? It's a counter and not a gauge. I assumed that it was meant to be a counter that spanned the life of the vtgate as the description is "Number of vstreams that ended with errors".

[twthorn]

Ah yes good point, this could be called multiple times. So we will just keep with Add zero.

[mattlord]

Nice work on this, @twthorn ! ❤️ Much appreciated.

For docs, we should add these new metrics to the v22 docs here: https://vitess.io/docs/22.0/reference/vreplication/metrics/#vtgate-metrics

Do you mind opening a website/docs PR for that as well? I can help as needed.

[twthorn]

Sure thing, opened here vitessio/website#1948