feat: support more specific prefixes in ?collect parameter

[antoinedeschenes]

This changes the ?collect parameter filter to allow more specific prefixes than those setup in the metrics-prefixes.

Example use cases:

1. I have the redis.googleapis.com/stats/memory/usage_ratio prefix set in the ?collect config of dozens of projects.

Now, I'd like to enable the redis.googleapis.com/stats/memory/usage metric in the stackdriver_exporter metric prefixes, but since it conflicts with the more-specific redis.googleapis.com/stats/memory/usage_ratio metric name, the exporter will stop accepting the ?collect param I've set in other configurations.

2. I want to whitelist all redis.googleapis.com metrics, but only collect 1 or 2 metrics with ?collect. Currently, the prefixes have to be exact matches so this is not possible without the proposed change.

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To test:

$ go run . --google.project-id=<gcp-project-name> --monitoring.metrics-prefixes=redis.googleapis.com/stats/
$ curl http://localhost:9255/metrics?collect=redis.googleapis.com/stats/memory/usage/ | grep stackdriver_redis

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Other question: is there a technical reason that would prevent from having a flag disabling prefix filtering in ?collect?

[antoinedeschenes]

cc: @SuperQ @kgeckhart

[antoinedeschenes]

Looks like that feature was already implemented along another change and then reverted through PR #316

though the reason for the revert seems to be that the main handler was removed, which caused a regression to aggregate-deltas #315

[kgeckhart]

Other question: is there a technical reason that would prevent from having a flag disabling prefix filtering in ?collect?

The configuration could be a bit confusing if you have to provide metric prefixes that are completely ignored when calling collect.

Looks like that feature was already implemented along another change and then reverted through PR #316

though the reason for the revert seems to be that the main handler was removed, which caused a regression to aggregate-deltas #315

You're right, this implementation should be okay because it's not changing any of the handler functionality directly.

Thanks for doing this! I'll see if I can resolve ?collect being incompatible with aggregate deltas separately.

[antoinedeschenes]

The configuration could be a bit confusing if you have to provide metric prefixes that are completely ignored when calling collect.

Oh, by that I mean that ?collect could allow to specify metric prefixes that aren't specified in the exporter command-line arguments