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.

[sed-i]

Looks good!

For quick reference in the future, could you please add a "Summary" section with a table such as

	juju_unit	juju_application	juju_charm	juju_model	juju_model_uuid
cos-config
gagent:juju-info
gagent:cos-agent
...

comparing which labels are present on which relations, and potentially contrasting label matchers and alert labels.

[Copilot]

Pull request overview

Documents how the prometheus_scrape library injects the juju_unit topology label into Prometheus scrape jobs for both wildcard and explicit (non-wildcard) scrape targets.

Changes:

• Expanded juju-topology-labels.md to describe juju_unit injection behavior for wildcard vs non-wildcard scrape targets under metrics-endpoint.
• Updated the “Exposing a Metrics Endpoint” how-to with guidance and an example for using explicit FQDN/IP targets (e.g., for strict TLS/SNI).
• Minor Markdown whitespace/formatting cleanup in the affected sections.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
docs/reference/juju-topology-labels.md	Adds reference documentation for juju_unit injection behavior for wildcard and non-wildcard scrape targets.
docs/how-to/exposing-a-metrics-endpoint.md	Adds author guidance and an example showing explicit targets and how juju_unit enrichment applies.

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[Copilot]

The statement that Prometheus "expands" the * target is inaccurate: Prometheus treats static_configs.targets as literal host:port strings and does not expand *. The per-unit expansion/juju_unit enrichment is done before Prometheus receives the final scrape config (e.g., by the prometheus_scrape library / charm generating the jobs). Please reword to avoid attributing this behavior to Prometheus itself.

Suggested change

	The `*` wildcard in the target address is the most common pattern. At scrape
	time, Prometheus expands it to one scrape job per unit, each labeled with the
	corresponding `juju_unit` topology label.
	The `*` wildcard in the target address is the most common pattern. The
	`prometheus_scrape` library uses it to generate one scrape target per unit
	before Prometheus receives the final scrape configuration, and each target is
	labeled with the corresponding `juju_unit` topology label.