Fix Unicorn not cleaning up stale metrics file at startup
before_exec hook in Unicorn causes GitLab to clean up
metrics files, but that hook does not run if Unicorn if
starts up anew.
before_master_fork hook or something similar, but
shows an example of how to do the equivalent using a flag inside the
We now use this
run_once variable to clean up Prometheus metric files
before running the master.
Closes gitlab-ce#62715 (closed)