Allow cert contents in prometheus scrape config tls config

[charleshansen]

For context, on the Bosh team, we are trying to deploy a prometheus in our acceptance environment to try out providing our own metrics. (the bosh-exporter is cool, but very expensive to run, metrics provided directly by Bosh should be much cheaper). The bosh metrics endpoint is secured with mTLS and we need to give certs to the scrape config.

Thanks,
Charles

PR details:

Prometheus wants scrape_config tls_config to use file paths, but it is difficult
to directly put certificates on a vm while deploying with bosh.

If cert_contents are provided, the pre-start script will write the
contents into cert files that prometheus can use when it starts

Prometheus validates that there are not extra keys in its configuration,
so the extra cert contents keys need to be removed.

#169551239

Co-authored-by: Conor Nosal [email protected]

[charleshansen]

For reference, an updated scrape_configs: looks like

scrape_configs:
  - job_name: prometheus
     static_configs:
    - targets:
       - localhost:9090
  - job_name: bosh
    static_configs:
    - targets:
      - 10.0.0.6:9091
      scheme: https
      tls_config:
        ca_file: /var/vcap/jobs/prometheus2/certs/ca.cert
        ca_contents: ((metrics_server_client_tls.ca))
        cert_file: /var/vcap/jobs/prometheus2/certs/cert.cert
        cert_contents: ((metrics_server_client_tls.certificate))
        key_file: /var/vcap/jobs/prometheus2/certs/key.cert
        key_contents: ((metrics_server_client_tls.private_key))

[frodenas]

Thanks @charleshansen! And apologies for the delay merging it :(