Promtail: allow single job with multiple service discovery elements

[rfratto]

Users can't configure a single job with two different service discovery entries:

- job_name: job
  static_configs:
    - targets:
        - localhost
      labels:
        name: a
        job: job
        __path__: /data/file.a.log
    - targets:
        - localhost
      labels:
        name: b
        job: job
        __path__: /data/file.b.log

When using this config, Promtail will only use the latest element in the array. I believe
these lines are the cause.

[adityacs]

@rfratto This is a bug coming from here https://github.com/prometheus/prometheus/blob/master/discovery/manager.go#L301. We are missing Source value in our configs. Which in turn ends up with targetGroups containing the last element of the array.

Trying to configure with source value gives Unmarshal error which may be due to https://github.com/prometheus/prometheus/blob/master/discovery/targetgroup/targetgroup.go#L41-L43

The thing is other SD configs like DNS, file etc.. Source is an extracted value. However, for static_configs there is no source extracted.

I am working to fix this.