Add HandlerWithAllowlist which can control exposed metrics

[CatherineF-dev]

After adding HandlerWithAllowlist, cilium or other components can exclude high cardinality metrics instead of disabling metrics collection in a running cluster.

context: cilium/cilium#27471

I notice this pr introduces a new dependency (dto "github.com/prometheus/client_model/go"), maybe need to move HandlerWithAllowlist to another file.

[bwplotka]

Thanks!

Normally this could be done on registration, so you only register what you want.

I guess you want to register all and filter afterwards in one place. This has consequences, one would check in the code that certain metric is instrumented but it's actually ignored near the handler.

Still, I think it's fine, however there is no need to add complexity to handler AND add new gatherer. We could simply add new gatherer similar to what you have here. Anyone can then wrap their registry with it.

[bwplotka]

I would just add FilteredGatherer that wraps registry and pass through only matching metrics. Matchers can be done with something like https://github.com/prometheus/client_golang/blob/main/prometheus/collectors/go_collector_latest.go#L93 - regexp.

This allows us to allow list those metrics you wanted to hardcode here, outside of FilteredGatherer - on caller side (: