qscheduler: use gauge instead of counter for queue_size

BUG=None

Change-Id: I32b63e90926fc43fdc015e647efc8a45014934a1
Reviewed-on: https://chromium-review.googlesource.com/c/1478020
Auto-Submit: Aviv Keshet <akeshet@chromium.org>
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#20766}
diff --git a/go/src/infra/appengine/qscheduler-swarming/app/state/metrics.go b/go/src/infra/appengine/qscheduler-swarming/app/state/metrics.go
index 604e036..f1846b9 100644
--- a/go/src/infra/appengine/qscheduler-swarming/app/state/metrics.go
+++ b/go/src/infra/appengine/qscheduler-swarming/app/state/metrics.go
@@ -69,7 +69,7 @@
 		field.String("account_id"),
 	)
 
-	gaugeQueueSize = metric.NewCounter(
+	gaugeQueueSize = metric.NewInt(
 		"qscheduler/state/queue_size",
 		"The number of tasks in the queue.",
 		nil,