Add missing circuitbreaker metrics to prometheus documentation

doc/administration/monitoring/prometheus/gitlab_metrics.md

@@ -45,6 +45,8 @@ In this experimental phase, only a few metrics are available:
 | redis_ping_success                | Gauge     | 9.4   | Whether or not the last redis ping succeeded |
 | redis_ping_latency_seconds        | Gauge     | 9.4   | Round trip time of the redis ping |
 | user_session_logins_total         | Counter   | 9.4   | Counter of how many users have logged in |
+| filesystem_circuitbreaker_latency_seconds | Histogram | 9.5 | Latency of the stat check the circuitbreaker uses to probe a shard |
+| filesystem_circuitbreaker         | Gauge     | 9.5   | Wether or not the circuit for a certain shard is broken or not |
 
 ## Metrics shared directory