[prometheus] Increase retentionSize

This ups prometheus's retentionSize.  Now that we're on 2.15,
retentionSize needs to include both the tsdb and the WAL.

We really don't want it to run out of disk again though so we're
leaving a generous headroom 😄

Author: philandstuff

charts/gsp-cluster/charts/gsp-monitoring/values.yaml

@@ -18,7 +18,7 @@ prometheus-operator:
         product: local
       replicas: 2
       retention: "60d"
-      retentionSize: "190GB"
+      retentionSize: "350GB"
       walCompression: true
       ruleSelectorNilUsesHelmValues: false
       ruleSelector: {}