Allow override of sentry dsn by setting it on disk
It would be useful to use an environment or config override for the Sentry DSN. This is so that we can track errors separately for environments like canary that are sharing the same database as production. This would also be useful for the geo feature, and was previously discussed in https://gitlab.com/gitlab-org/gitlab-ee/issues/4509 for having an override table for application settings. I think something simple like an environment variable that overrides the sentry DSN would be nice in the shortterm for canary.
cc @nick.thomas since this also came up for the geo feature.