Promtail: add consul agent service discovery

[trevorwhitney]

What this PR does / why we need it:
This PR adds a new service discovery option to promtail, which is an adaptation of the existing prometheus consul service discovery to use the consul agent APIs instead. This is beneficial for very large consul clusters, where the catalog api endpoints can get very big and slow. Since promtail is often deployed on every node in a cluster, it makes sense for it be able to leverage the local consul agent which is also often deployed on every node in a cluster.

Which issue(s) this PR fixes:
Fixes #3744

Special notes for your reviewer:
This is running in a test environment on GCP if you'd like to poke at it

Checklist

• Documentation added
• Tests updated

[KMiller-Grafana]

I have only reviewed the wording of the documentation. I have not reviewed the code. With corrections, I approve the documentation of this feature.

[slim-bean]

Looks Good!

[owen-d]

I skimmed it; looks pretty good to me. I'd probably prefer we use consul_agent instead of consulagent in our yaml configs & exposed meta labels, but that's largely a nitpick.

[owen-d]

Part of me would like to see this access protected against panics, but I see the method signature doesn't support an error return argument. We could use logging + failure metrics here, but the panic on such an unexpected code path may be fine for now -- it's a quicker, cruder way to surface problems.

[slim-bean]

I skimmed it; looks pretty good to me. I'd probably prefer we use consul_agent instead of consulagent in our yaml configs & exposed meta labels, but that's largely a nitpick.

I think this is a change now or change never kind of thing @owen-d, I merged as is because there is an existing precedent with digitalocean. Not a great argument per say but I guess one of the boards on the bike shed was already this color :)