Add support for AuthSecret from file
related to: https://github.com/prometheus/alertmanager/issues/2498
Something's weird with CircleCI reporting Could not find a usable config.yml, you may have revoked the CircleCI OAuth app.... Have you enabled CircleCI on your fork repository?
I am not aware of doing any Circle CI related stuff on my fork. I just forked and worked on the fix.
Documentation is not up to date with regards to auth_password_file nor auth_secret_file.
@Git-Jiro can you rebase this PR?
@TheMeier sure!
@Git-Jiro can you also pleas sign-off your commits. See https://github.com/prometheus/alertmanager/pull/3087/checks?check_run_id=55492475887