[prometheusexporter] Expose job and instance labels

[gouthamve]

NOTE: This is WIP PR and I wanted to understand if my approach is right before I write the tests for this.

Description:

Fixes #8355

Testing: I've run the collector with the following config and the exposed metrics had job and instance attached to them:

receivers:
  prometheus:
    config:
      scrape_configs:
      - job_name: 'beep-boop'
        scrape_interval: 5s
        static_configs:
        - targets: ['localhost:9090']

processors:
  batch:

exporters:
  logging:
    logLevel: info

  prometheus:
    endpoint: localhost:9091

service:
  pipelines:
    metrics:
      receivers: [prometheus]
      processors: [batch]
      exporters: [prometheus]

[gouthamve]

@dashpole Could you look at my approach of plumbing the resource attributes all the way down, and let me know if it looks okay? If it does, I will work on adding tests.

[dashpole]

It looks good to me!

[gouthamve]

I've added the tests and the tests caught a bug 😀 @dashpole I think this is ready for review!

[Mario-Hofstaetter]

@gouthamve Could you please update https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main/exporter/prometheusexporter/README.md to explain the behavior regarding new job and instance labels?
Is job_name from scrape_configs used as job ?

I have added those labels myself in a way so have to be careful not to break anything.

[gouthamve]

Is job_name from scrape_configs used as job ?

Yes. I'll update the docs now.