[receiver/prometheus] Investigate if we can relax usage of fallback_scrape_protocol

[ArthurSens]

Component(s)

receiver/prometheus

Describe the issue you're reporting

During #36873, we faced a lot of failing tests because of content type negotiation failures. To update things quickly, we ended up adding a fallback_scrape_protocol option everywhere, but it's not clear to us if that's the best option.

Let's investigate and clean up where we can

[github-actions]

Pinging code owners:

• receiver/prometheus: @Aneurysm9 @dashpole

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[crobert-1]

Removing needs triage based on response from code owner, with the understanding that no conclusion has been reached.