fix: correct Redis client counting metric

[carodewig]

This PR removes the apollo.router.cache.redis.connections metric and replaces it with a apollo.router.cache.redis.clients.

The connections metric was implemented with an up-down counter which would sometimes not be collected properly (i.e. it could go negative). The name *.connections was also inaccurate given that our Redis clients will each make multiple connections, one to each node in the Redis pool (if in clustered mode).

The new clients metric counts the number of clients across the router via an AtomicU64 and surfaces that value in a gauge.

Caveat: the old metric included a kind attribute to reflect the number of clients in each pool (ie entity caching, query planning). The new metric does not include this attribute as doing so would require we store a global Arc<RwLock<HashMap>> (to be able to track each pool type separately). I'm happy to change the implementation to do this, but as the purpose of the metric is to make sure the number of clients isn't growing unbounded (#7319), it seemed unnecessary.

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[apollo-librarian]

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File Changes

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* graphos/routing/(latest)/observability/telemetry/instrumentation/standard-instruments.mdx

Build ID: 39049c2d889800e1a1c8ef5a
Build Logs: View logs

URL: https://www.apollographql.com/docs/deploy-preview/39049c2d889800e1a1c8ef5a

[BrynCooke]

Needs docs and changelog, but otherwise it looks OK.

[BrynCooke]

Nit: It would be nice if the sub was in a Drop implementation.

[carodewig]

I don't think I can put it in the DropSafeRedisPool Drop implementation, but I think this accomplishes the same thing: 547bd2a

[carodewig]

Comment for posterity: I think having the decrement in a Drop implementation would actually make the metric not useful, as the behavior in #7319 was that dropping the client didn't actually terminate the connection.

[carodewig]

@BrynCooke Does #8174 eliminate the need for this PR? I prefer the existing UpDownCounter implementation to the global I had to add here.

[BrynCooke]

@BrynCooke Does #8174 eliminate the need for this PR? I prefer the existing UpDownCounter implementation to the global I had to add here.

Not yet, there are other followup PRs. I'll deal with the conversion from gauges back to updown counters separately.

[BrynCooke]

Do docs need updating?

[carodewig]

@BrynCooke Yep, good catch!

I marked the changeset as a feat because we're adding a metric and removing another, should it be marked as fix or breaking instead?

[BrynCooke]

Seeing as the old metric was broken it can be a fix, but make sure to include wording in the changelog as to why the old metric has gone and what users need to do.

Unit added to metric