Instrument authentication via ldap_with_git

Related to #546680 (closed)

What does this MR do and why?

Instruments the number of unique users who try to authenticate with Git over HTTP(S) using LDAP credentials, when the prevent_ldap_sign_in? admin setting on self-managed is enabled.

How to set up and validate locally

1. Setup the interval events monitor

• Run this in a terminal after you cd into gitlab-development-kit/gitlab: rails runner scripts/internal_events/monitor.rb authenticate_to_ldap_with_git_over_https_when_prevent_ldap_sign_in_is_enabled
• If the event monitor does not work, use the rails console

gdk rails console
require_relative 'spec/support/helpers/service_ping_helpers.rb'
ServicePingHelpers.get_current_usage_metric_value('counts.count_total_authenticate_to_ldap_with_git_over_https_when_prevent_ldap_sign_in_is_enabled_monthly')
ServicePingHelpers.get_current_usage_metric_value('counts.count_total_authenticate_to_ldap_with_git_over_https_when_prevent_ldap_sign_in_is_enabled_weekly')

2. Setup OpenLDAP. Run bundle exec rake gitlab:ldap:check in a terminal to verify that at least 1 LDAP server is running.
3. Login with one of the autocreated users like: username: john, password: password
4. Create a private project & try to clone it with git via https like git clone https://gdk.test:3443/john/johnldap_private.git. Verify the events & metrics don't increment
5. Create a second private project
6. Disable LDAP web sign in & run gdk reconfigure && gdk restart.
7. Try to clone the second project with git via https, like git clone https://gdk.test:3443/john/johnldap_private2.git. Verify the events & metrics increment per unique user

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