[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