Optimize ldap_keys count in usage_data

# 1. https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/ee/gitlab/usage_data.rb#L149 
ldap_keys: count(::LDAPKey)

# 2. https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/ee/gitlab/usage_data.rb#L245
ldap_keys: distinct_count(::LDAPKey.where(time_period), :user_id)

# 3. https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/ee/gitlab/usage_data.rb#L245
time_period = { created_at: 28.days.ago..Time.current }
ldap_keys: distinct_count(::LDAPKey.where(time_period), :user_id),