[stable/prometheus] default retention time is 15 days if not specified (#13965)

* If you do not specify a retention time then by default it is set to 15 days.
In this PR I make that the default so that its clear to users.

Signed-off-by: David O'Dell <dave@helix.re>

* bumped chart version

Signed-off-by: David O'Dell <dave@helix.re>

* rev'd chart version to 8.11.5

Signed-off-by: David O'Dell <dave@helix.re>

stable/prometheus/README.md

@@ -303,7 +303,7 @@ Parameter | Description | Default
 `serviceAccounts.server.create` | If true, create the server service account | `true`
 `serviceAccounts.server.name` | name of the server service account to use or create | `{{ prometheus.server.fullname }}`
 `server.terminationGracePeriodSeconds` | Prometheus server Pod termination grace period | `300`
-`server.retention` | (optional) Prometheus data retention | `""`
+`server.retention` | (optional) Prometheus data retention | `"15d"`
 `serverFiles.alerts` | Prometheus server alerts configuration | `{}`
 `serverFiles.rules` | Prometheus server rules configuration | `{}`
 `serverFiles.prometheus.yml` | Prometheus server scrape configuration | example configuration

stable/prometheus/values.yaml

@@ -778,9 +778,9 @@ server:
   ##
   terminationGracePeriodSeconds: 300
 
-  ## Prometheus data retention period (i.e 360h)
+  ## Prometheus data retention period (default if not specified is 15 days)
   ##
-  retention: ""
+  retention: "15d"
 
 pushgateway:
   ## If false, pushgateway will not be installed