Adding the feature to launch Prometheus process with custom script

This change adds feature to launch Prometheus process using a custom script which should be stored in override values. Because the known issue https://github.com/prometheus/prometheus/issues/6934 is still open many years, we are going to struggle with growing WAL files using our custom downstream wrapper script which stops Prometheus process and deletes WALs.
This solution can not fit all customers because completely kills wal cached data but it is ok for our purposes. Such way I just added the feature to use another custom script to launch Prometheus and left original functionality by default. Default/custom mode are defined in 'values.yaml' as the body of the custom launcher script.

Change-Id: Ie02ea1d6a7de5c676e2e96f3dcd6aca172af4afb

prometheus/Chart.yaml

@@ -15,7 +15,7 @@ apiVersion: v1
 appVersion: v2.25.0
 description: OpenStack-Helm Prometheus
 name: prometheus
-version: 0.1.13
+version: 0.1.14
 home: https://prometheus.io/
 sources:
   - https://github.com/prometheus/prometheus

prometheus/templates/bin/_prometheus.sh.tpl

@@ -15,15 +15,23 @@ limitations under the License.
 */}}
 
 set -ex
-COMMAND="${@:-start}"
 
-function start () {
-{{ $flags := include "prometheus.utils.command_line_flags" .Values.conf.prometheus.command_line_flags }}
-  exec /bin/prometheus --config.file=/etc/config/prometheus.yml {{ $flags }}
-}
+# Two ways how to launch init process in container: by default and custom (defined in override values).
+{{ $deflaunch := .Values.proc_launch.prometheus.default }}
+if [ "{{ $deflaunch }}" = true ]
+then
+  COMMAND="${@:-start}"
 
-function stop () {
-  kill -TERM 1
-}
+  function start () {
+  {{ $flags := include "prometheus.utils.command_line_flags" .Values.conf.prometheus.command_line_flags }}
+    exec /bin/prometheus --config.file=/etc/config/prometheus.yml {{ $flags }}
+  }
 
-$COMMAND
+  function stop () {
+    kill -TERM 1
+  }
+
+  $COMMAND
+else
+  {{ tpl (.Values.proc_launch.prometheus.custom_launch) . }}
+fi

prometheus/values.yaml

@@ -271,6 +271,17 @@ network_policy:
     egress:
       - {}
 
+proc_launch:
+  prometheus:
+    default: true
+    custom_launch: |
+        while true
+        do
+          echo "If 'proc_launch.prometheus.default: false'."
+          echo "Your custom shell script code you can put here."
+          sleep 10
+        done
+
 secrets:
   oci_image_registry:
     prometheus: prometheus-oci-image-registry-key

releasenotes/notes/prometheus.yaml

@@ -14,4 +14,5 @@ prometheus:
   - 0.1.11 Update htk requirements
   - 0.1.12 Update default image value to Wallaby
   - 0.1.13 Added OCI registry authentication
+  - 0.1.14 Added feature to launch Prometheus with custom script
 ...