Ignore "not our ref" errors from gitlab-sshd error metrics

If a client requests a ref that cannot be found in the repository,
previously gitlab-sshd would record it as part of its service level
indicator metric. This is really an application error between the
client and the Git repository, so we exclude it from our metrics.

Relates to
https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/15848

Changelog: fixed