Rename Prometheus alerts for consistency

Change-Id: I96fb789bf73af22d56fc6c6980626647f87409d4
diff --git a/prometheus/meta/prometheus.yml b/prometheus/meta/prometheus.yml
index eb8df8d..07eb726 100644
--- a/prometheus/meta/prometheus.yml
+++ b/prometheus/meta/prometheus.yml
@@ -1,7 +1,7 @@
 {% raw %}
 server:
   alert:
-    PrometheusUP:
+    PrometheusTargetDown:
       if: 'up != 1'
       labels:
         severity: critical