Clarify ScrapeConfig label selection

[pavangudiwada]

Description

This PR clarifies how to configure the Prometheus Operator to select ScrapeConfigs from multiple namespaces. It also shows the field scrapeConfigNamespaceSelector.

Type of change

What type of changes does your code introduce to the Prometheus operator? Put an x in the box that apply.

• CHANGE (fix or feature that would cause existing functionality to not work as expected)
• FEATURE (non-breaking change which adds functionality)
• BUGFIX (non-breaking change which fixes an issue)
• ENHANCEMENT (non-breaking change which improves existing functionality)
• NONE (if none of the other choices apply. Example, tooling, build system, CI, docs, etc.)

Verification

Please check the Prometheus-Operator testing guidelines for recommendations about automated tests.

Changelog entry

Please put a one-line changelog entry below. This will be copied to the changelog file during the release process.

Docs clarifying ScrapeConfig label selection

[slashpai]

May be also show a example with selecting based on namespace labels?

[pavangudiwada]

@slashpai let me know if the following works better

1. Changing Both the Prometheus and PrometheusAgent CRD have a scrapeConfigSelector field to Both the Prometheus and PrometheusAgent CRD have scrapeConfigNamespaceSelector and scrapeConfigSelector fields.
2. Add example to configure both fields with labels.
3. Another example to show them set to {} to select all selectors from all namespaces.

Question: In the context of #6404 While (3) solves the case of selecting all namespaces/objects, what does selecting none look like? Do we do something like below to add non-existing random label?

spec:
  scrapeConfigSelector:
    matchLabels:
      nolabel: non-existing-label

[ArthurSens]

Suggested change

	To select `ScrapeConfigs` based on the namespace `scrapeConfigNamespaceSelector` field can be used. Set it to `{}` to match `ScrapeConfigs` from all namespaces.
	To select `ScrapeConfigs` based on the namespace, `scrapeConfigNamespaceSelector` field can be used. Set it to `{}` to match `ScrapeConfigs` from all namespaces.