Fix single job with multiple service discovery elements

[adityacs]

What this PR does / why we need it:
Fix single job with multiple service discovery elements

Which issue(s) this PR fixes:
Fixes #1754

@rfratto Kindly review

[rfratto]

Thank you for looking into this @adityacs! This LGTM aside from my one nit.

[rfratto]

LGTM, thanks again! Waiting for CI to pass and I'll merge.

[zswanson]

@adityacs @rfratto this seems to have regressed, I'm seeing the same issue in promtail 1.6.0 with a similar config as the original issue.

scrape_configs:
  - job_name: centos
    pipeline_stages: [<stuff>]
    static_configs:
      - targets: ['localhost']
        labels:
          job: varlogs
          __path__: /var/log/messages
      - targets: ['localhost']
        labels:
          job: varlogs
          __path__: /var/log/secure

[adityacs]

@zswanson This is not a regression. You have to make sure the labels are unique. Try following configuration

static_configs:
      - targets: ['localhost']
        labels:
          job: varlogs
          log_name: messages
          __path__: /var/log/messages
      - targets: ['localhost']
        labels:
          job: varlogs
          log_name: secure_log
          __path__: /var/log/secure

[zswanson]

I see, ok. I thought that the __path__ internal label, that gets translated to filename when stored in loki, handled that uniqueness.