Add documentation for trace counter metric

doc/administration/monitoring/prometheus/gitlab_metrics.md

@@ -140,6 +140,7 @@ The following metrics are available:
 | `gitlab_snowplow_events_total`                                   | Counter     | 14.1    | Total number of GitLab Snowplow product intelligence events emitted                                                   |                                                           |
 | `gitlab_snowplow_failed_events_total`                            | Counter     | 14.1    | Total number of GitLab Snowplow product intelligence events emission failures                                         |                                                           |
 | `gitlab_snowplow_successful_events_total`                        | Counter     | 14.1    | Total number of GitLab Snowplow product intelligence events emission successes                                        |                                                           |
+| `gitlab_ci_build_trace_errors_total`                             | Counter     | 14.4    | Total amount of different error types on a build trace                                                                | `type`                                                    |
 
 ## Metrics controlled by a feature flag