Alerting: Add metric counting rule groups per org

[alexweav]

What is this feature?

Adds a metric, grafana_alerting_rule_groups, which tracks rule groups.
The current metrics emitted by the scheduler only track rules.

This will also inform the cardinality impact of adding rule_group as a label, like how Prometheus works.

Why do we need this feature?
n/a

Who is this feature for?
operators, anyone observing Grafana

Which issue(s) does this PR fix?:

n/a

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.