Implement Prometheus-style recording rules

[dannykopping]

Is your feature request related to a problem? Please describe.
Loki currently supports Prometheus-style alert rules. Prometheus also supports recording rules, and Loki should implement this functionality too.

This feature will allow users to produce Prometheus metrics from logs.

Describe the solution you'd like
Loki should evaluate recording rules using LogQL, and make the resulting samples available to Prometheus (or Cortex/Thanos).

Describe alternatives you've considered
Currently there is no other mechanism to generate Prometheus metrics from logs.

Additional context
N/A