validate promtail config via a dedicated flag

[landryb]

as was done for loki in #2737 for #2713 it would be great if promtail had a flag to just validate the config file and exit (with the corresponding exit status). That would allow for seamless deployment/validation of promtail configs by ansible for example.

for example, i have this for loki:

- name: Template loki configuration
  template:
    src: config.yml.j2
    dest: /opt/loki/config.yml
    validate: "/opt/loki/loki-linux-amd64 -verify-config -config.file %s"
  notify: restart loki

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 30 days. It will be closed in 7 days if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 30 days. It will be closed in 7 days if no further activity occurs. Thank you for your contributions.