stable/prometheus: Update default version to 2.2.1 (#4288)

stable/prometheus/Chart.yaml

@@ -1,5 +1,5 @@
 name: prometheus
-version: 5.4.4
+version: 5.5.0
 description: Prometheus is a monitoring system and time series database.
 home: https://prometheus.io/
 icon: https://raw.githubusercontent.com/prometheus/prometheus.github.io/master/assets/prometheus_logo-cb55bb5c346.png

stable/prometheus/values.yaml

@@ -388,7 +388,7 @@ server:
   ##
   image:
     repository: prom/prometheus
-    tag: v2.1.0
+    tag: v2.2.1
     pullPolicy: IfNotPresent
 
   ## The URL prefix at which the container can be accessed. Useful in the case the '-web.external-url' includes a slug