docs: document juju_unit label injection for wildcard and non-wildcard scrape targets

[Abuelodelanada]

Issue

Document how the prometheus_scrape library injects the juju_unit topology label into Prometheus scrape jobs, covering both wildcard and non-wildcard targets.

Checklist

• I have added or updated relevant documentation.
• PR title makes an appropriate release note and follows conventional commits syntax.
• Merge target is the correct branch, and relevant tandem backport PRs opened.

[Copilot]

Pull request overview

This PR updates the documentation to explain how the prometheus_scrape library injects the juju_unit topology label into Prometheus scrape jobs for both wildcard and non-wildcard scrape targets, helping users understand when per-unit labeling is (and isn’t) applied.

Changes:

• Document juju_unit label injection behavior for wildcard vs non-wildcard scrape targets under metrics-endpoint.
• Extend the “exposing a metrics endpoint” how-to with guidance and an example for explicit (non-wildcard) targets.
• Clean up minor Markdown formatting/indentation in the topology labels reference doc.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
docs/reference/juju-topology-labels.md	Adds a new “metrics-endpoint” subsection describing juju_unit injection behavior for wildcard and non-wildcard scrape targets.
docs/how-to/exposing-a-metrics-endpoint.md	Adds guidance and a code example for using explicit non-wildcard targets and explains juju_unit enrichment behavior.

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[YanisaHS]

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