Merge branch 'docs-job-waiter-metrics' into 'master'

Document JobWaiter metrics

Closes gitlab-com/gl-infra/scalability#166

See merge request gitlab-org/gitlab!26159

doc/administration/monitoring/prometheus/gitlab_metrics.md

@@ -30,6 +30,8 @@ The following metrics are available:
 | `gitlab_cache_misses_total`                                    | Counter   |                   10.2 | Cache read miss                                                                                     | controller, action                                  |
 | `gitlab_cache_operation_duration_seconds`                      | Histogram |                   10.2 | Cache access time                                                                                   |                                                     |
 | `gitlab_cache_operations_total`                                | Counter   |                   12.2 | Cache operations by controller/action                                                               | controller, action, operation                       |
+| `job_waiter_started_total`                                     | Counter   |                   12.9 | Number of batches of jobs started where a web request is waiting for the jobs to complete           | worker                                              |
+| `job_waiter_timeouts_total`                                    | Counter   |                   12.9 | Number of batches of jobs that timed out where a web request is waiting for the jobs to complete    | worker                                              |
 | `gitlab_database_transaction_seconds`                          | Histogram |                   12.1 | Time spent in database transactions, in seconds                                                     |                                                     |
 | `gitlab_method_call_duration_seconds`                          | Histogram |                   10.2 | Method calls real duration                                                                          | controller, action, module, method                  |
 | `gitlab_page_out_of_bounds`                                    | Counter   |                   12.8 | Counter for the PageLimiter pagination limit being hit                                              | controller, action, bot                             |