Add missing pipeline size metric label

doc/administration/monitoring/prometheus/gitlab_metrics.md

@@ -43,7 +43,7 @@ The following metrics are available:
 | `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`                 |
 | `gitlab_ci_pipeline_creation_duration_seconds`                 | Histogram |                   13.0 | Time in seconds it takes to create a CI/CD pipeline                                                 |                                                     |
-| `gitlab_ci_pipeline_size_builds`                               | Histogram |                   13.1 | Total number of builds within a pipeline grouped by a pipeline source                               |                                                     |
+| `gitlab_ci_pipeline_size_builds`                               | Histogram |                   13.1 | Total number of builds within a pipeline grouped by a pipeline source                               | `source`                                            |
 | `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                                                     |                                                     |