Emit tasks queue metric when leaving the queue.

This is necessary to report the length of the queues
as zero when the metric is not present (e.g. when there
are no tasks in a given queue).

Signed-off-by: Alessandro Degano <[email protected]>

atc/worker/client.go

@@ -332,7 +332,7 @@ func (client *client) chooseTaskWorker(
 				workerTags := strings.Join(tags, "_")
 				queue_position, qlen, err := client.taskQueue.FindOrAppend(taskId, workerSpec.Platform, teamId, workerTags, logger)
 				if !alreadyQueued {
-					defer client.taskQueue.Dequeue(taskId, logger)
+					defer dequeue(client.taskQueue, taskId, workerSpec.Platform, teamId, workerTags, logger)
 					alreadyQueued = true
 				}
 				if err != nil {
@@ -419,7 +419,6 @@ func (client *client) chooseTaskWorker(
 				}
 			}
 		}
-
 		break
 	}
 
@@ -434,6 +433,21 @@ func decreaseActiveTasks(logger lager.Logger, w Worker) {
 	}
 }
 
+func dequeue(queue db.TaskQueue, taskId string, platform string, teamId int, workerTags string, logger lager.Logger) {
+	queue.Dequeue(taskId, logger)
+	qlen, err := queue.Length(taskId)
+	if err != nil {
+		logger.Error("failed-to-fetch-queue-length", err)
+		return
+	}
+	metric.TaskQueue{
+		Length:     qlen,
+		Platform:   platform,
+		Team:       teamId,
+		WorkerTags: workerTags,
+	}.Emit(logger)
+}
+
 type processStatus struct {
 	processStatus int
 	processErr    error