[receiver/prometheus] Split target allocator into internal package

[mx-psi]

Component(s)

receiver/prometheus

Describe the issue you're reporting

The target allocator from the Prometheus receiver is a part of the codebase that not all contributors may be as familiar with as other parts. In the OpenTelemetry Operator, the target allocator is owned by a separate group of maintainers.

After discussion with the current set of codeowners of the receiver, this issue proposes splitting off this part of the codebase into an internal package (or a separate module under the top-level internal/ folder if there is a possibility of reuse), so that we can find new owners for this part of the codebase and ensure we can appropriately handle PRs and issues related to this part of the code.

cc @dashpole @Aneurysm9

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Pinging code owners:

• receiver/prometheus: @Aneurysm9 @dashpole

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[dashpole]

I support this change

[swiatekm]

I agree in spirit. The target allocator integration should be owned by the codeowners of the target allocator itself, which in principle includes some combination of @open-telemetry/operator-ta-maintainers and @open-telemetry/operator-maintainers.

I don't know if I like the idea of making it a separate package. It would only have a single consumer, with no real prospects of having more in the future, and would probably be heavily coupled to the prometheus receiver internals anyway. Do we have the option of moving to to a separate Go file, and having different codeowners just for that file?

[mx-psi]

I don't know if I like the idea of making it a separate package. It would only have a single consumer, with no real prospects of having more in the future, and would probably be heavily coupled to the prometheus receiver internals anyway. Do we have the option of moving to to a separate Go file, and having different codeowners just for that file?

A separate package could just be a receiver/prometheusreceiver/internal/targetallocator folder, no need for it to be exposed to other modules if there's a single consumer. A single file is also fine (although I suppose at least two files are needed, one of them for testing)

[dashpole]

In my current PoC, it is 2 files + 2 test files, with the configuration being split into a config.go file. I'll push what I have tomorrow if I can

[dashpole]

https://github.com/open-telemetry/opentelemetry-collector-contrib/pull/33223

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