Align default labels between loki, prometheus, and pyroscope components

[rfratto]

Request

The prometheus.scrape component adds two default labels to every metric: instance (the address of the target) and job (a descriptor for the thing being scraped, which defaults to the Alloy component ID).

Similar components (at least loki.source.file) do not have the same set of default labels.

We should:

• Identify a base set of labels that all scrape-based components should have
• Ensure that the base set of labels is attached to all targets
• Ensure that the base set of labels is documented

Use case

Aligning expectations between usages of loki.source.file/prometheus.scrape/pyroscope.scrape.

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