Added PROMETHEUS_USE_STATIC_DATADIR="true" for global

For prometheus running with global mode, we should use
the static data dir.

Change-Id: Ie57da07f8c9017617bf37728bd380e7fb25d9bc4
Related-Bug: PROD-18984
diff --git a/docker/swarm/stack/monitoring/prometheus/global.yml b/docker/swarm/stack/monitoring/prometheus/global.yml
index d86f456..6114f44 100644
--- a/docker/swarm/stack/monitoring/prometheus/global.yml
+++ b/docker/swarm/stack/monitoring/prometheus/global.yml
@@ -7,3 +7,5 @@
             server:
               deploy:
                 mode: global
+              environment:
+                PROMETHEUS_USE_STATIC_DATADIR: "true"