Docs: Add three Praefect metrics (gitaly_praefect_replication_latency_bucket,...

Docs: Add three Praefect metrics (gitaly_praefect_replication_latency_bucket, gitaly_praefect_replication_delay_bucket and gitaly_praefect_node_latency_bucket) to the docs

doc/administration/monitoring/prometheus/gitlab_metrics.md

@@ -139,6 +139,18 @@ The following metrics can be controlled by feature flags:
 | `gitlab_method_call_duration_seconds`                          | `prometheus_metrics_method_instrumentation`                        |
 | `gitlab_view_rendering_duration_seconds`                       | `prometheus_metrics_view_instrumentation`                          |
 
+## Praefect metrics
+
+You can [configure Praefect to report metrics](../../gitaly/praefect.md#praefect).
+These are some of the Praefect metrics served from the `/metrics` path on the [configured port](index.md#changing-the-port-and-address-prometheus-listens-on)
+(9652 by default).
+
+| Metric | Type | Since | Description | Labels |
+| :----- | :--- | ----: | :---------- | :----- |
+| `gitaly_praefect_replication_latency_bucket` | Histogram | 12.10 | The amount of time it takes for replication to complete once the replication job starts. |  |
+| `gitaly_praefect_replication_delay_bucket` | Histogram | 12.10 | A measure of how much time passes between when the replication job is created and when it starts. |  |
+| `gitaly_praefect_node_latency_bucket` | Histogram | 12.10 | The latency in Gitaly returning health check information to Praefect. This indicates Praefect connection saturation. |  |
+
 ## Sidekiq metrics
 
 Sidekiq jobs may also gather metrics, and these metrics can be accessed if the