Add `for` in gluster_up alert

Change-Id: If277d380a29060e6deb5c6b3ed650adb323e7d21
diff --git a/glusterfs/meta/prometheus.yml b/glusterfs/meta/prometheus.yml
index 30b4cd6..9a29d84 100644
--- a/glusterfs/meta/prometheus.yml
+++ b/glusterfs/meta/prometheus.yml
@@ -7,6 +7,7 @@
     GlusterFSDown:
       if: >-
         glusterfs_up != 1
+      for: 2m
       labels:
         severity: warning
         service: glusterfs