Add `spamcheck_latency_seconds` to documentation

doc/administration/monitoring/prometheus/gitlab_metrics.md

@@ -129,6 +129,7 @@ The following metrics are available:
 | `pipeline_graph_links_per_job_ratio`                             | Histogram   | 13.9    | Ratio of links to job per graph                                                                                       |                                                           |
 | `gitlab_ci_pipeline_security_orchestration_policy_processing_duration_seconds` | Histogram   | 13.12    | Time in seconds it takes to process Security Policies in CI/CD pipeline                                                                    |                                                           |
 | `gitlab_ci_difference_live_vs_actual_minutes`                    | Histogram   | 13.12    | Difference between CI minute consumption counted while jobs were running (live) vs when jobs are complete (actual). Used to enforce CI minute consumption limits on long running jobs. | `plan` |
+| `spamcheck_latency_seconds`                                      | Histogram   | 13.12   | The latency for requests between Rails and the anti-spam engine                                                        |                                                           |
 
 ## Metrics controlled by a feature flag