Be specific about the exposition formats supported

[alopezz]

What does this PR do?

Clarifies what formats are supported by the OpenMetrics check and how to configure which format is used.

Motivation

AI-2416.

The docs don't provide any clarity as to what the actual format that we support by default in the OpenMetrics check is and the name of the check (OpenMetrics, when we already have a check called Prometheus) doesn't help, considering we're actually assuming Prometheus format by default and not OpenMetrics. It's also not obvious how to enable OpenMetrics spec support (I'll be updating the config template to add some documentation in there as well).

Additional Notes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
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