Unify metrics to `gitlab_transaction_events_total`

The following discussion from gitlab-ce!32395 should be addressed:

From https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32395#note_211714193

@bjk-gitlab: These should be safe to enable, we removed the unsafe labels already.

I'm wondering if we should refactor these metrics slightly. Right now they are all separate metrics, but they seem like they might be better as a label.

Something like:

gitlab_transaction_events_total{event="push_tag"}
gitlab_transaction_events_total{event="update_build"}