There are situations where a container running Discourse may want to
cache the critical DNS services without running the cache_critical_dns
service, for example running migrations prior to running a full bore
application container.
Add a `--once` argument for the cache_critical_dns script that will
only execute the main loop once, and return the status code for the
script to use when exiting. 0 indicates no errors occured during SRV
resolution, and 1 indicates a failure during the SRV lookup.
Nothing is reported to prometheus in run_once mode. Generally this
mode of operation would be a part of a unix pipeline, in which the exit
status is a more meaningful and immediate signal than a prometheus metric.
The reporting has been moved into it's own method that can be called
only when the script is running as a service.
See /t/69597.
1867202a4d