Add metrics for ldap auth backend

Problem to solve

We should provide metrics for Ruby talking to LDAP. We should count requests and failures.

Intended users

GitLab Administrators

Further details

In order to alert GitLab instance admins that they are having LDAP auth problems, we need to track metrics for LDAP.

Proposal

Instrument counters in the LDAP auth controller.

Permissions and Security

N/A

Documentation

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.

What is the type of buyer?

Links / references