Loki: allow for multiple targets

[dannykopping]

Signed-off-by: Danny Kopping danny.kopping@grafana.com

What this PR does / why we need it:
Allows for multiple targets to be defined. A "target" is a module inside Loki (querier, ingester, etc), and currently we can only target a single one. If no -target argument is provided, all modules are loaded, so there should be no reason to not allow multiple targets (i.e. a subset of modules) to be defined.

Which issue(s) this PR fixes:
Fixes #3801

Special notes for your reviewer:
Cortex already implements this; I stole the implementation wholesale.

I also tried for a good couple hours to write tests for this, but unfortunately it's made rather difficult since a lot of the fields and functions in this package are unexported. However, Cortex does test this package well so I'm confident in the implementation.

I don't think the documentation needs updating, but I have modified the flag description.

Checklist

• Documentation added
• Tests updated

[cyriltovena]

LGTM however it should be noted that compactor should be ran as a singleton. The issue this closed can't run the compactor on multiple nodes.