Add metrics for ldap auth backend
Problem to solve
We should provide metrics for Ruby talking to LDAP. We should count requests and failures.
In order to alert GitLab instance admins that they are having LDAP auth problems, we need to track metrics for LDAP.
Instrument counters in the LDAP auth controller.
Permissions and Security
New metric names should be added to the metric documentation table.
What does success look like, and how can we measure that?
We can create an alerting rule for LDAP failures.