[mixin/alerts]: Enable configuring job prefix for alerts to prevent clashes with metrics from Loki/Tempo.

[mtweten]

What this PR does

This enables the ability to configure a job prefix for alerts in the mimir-mixin. Currently, some alerts just use a regex that matches any job prefix (e.g. .+/ or .*/). This can cause a problem because certain alerts will start triggering because they will match metrics for not just mimir, but also loki/tempo (if present).

As a specific example, GossipMembersTooHigh and RingMembersMismatch will start firing if Tempo metrics are being collected, since the current alerts will match not only on job=mimir/ingester, but also job=tempo/ingester

Typically, users would configure this alert prefix to the namespace mimir is deployed to, e.g. "mimir/".

This is a non-impactful change as it's a new configuration option which defaults to the previous prefix, .*/. However, it does change GossipMembersTooHigh from the prefix .+/ to .*/, but this shouldn't be relevant. Every other alert using this pattern was already using .*/ for the job prefix.

Which issue(s) this PR fixes or relates to

Fixes #5260

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• about-versioning.md updated with experimental features.