[prometheus] Respect storagePath value when deployed as StatefulSet (#1003)

As discussed in https://github.com/prometheus-community/helm-charts/issues/1002
the StatefulSet deploy option of Prometheus server previously ignored the
server.storagePath Helm value.

This patch fixes that behavior.

Signed-off-by: Jack Henschel <jackdev@mailbox.org>

charts/prometheus/Chart.yaml

@@ -1,7 +1,7 @@
 apiVersion: v2
 name: prometheus
 appVersion: 2.26.0
-version: 14.1.0
+version: 14.1.1
 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

charts/prometheus/templates/server/sts.yaml

@@ -90,7 +90,11 @@ spec:
             - --storage.tsdb.retention.time={{ .Values.server.retention }}
           {{- end }}
             - --config.file={{ .Values.server.configPath }}
+          {{- if .Values.server.storagePath }}
+            - --storage.tsdb.path={{ .Values.server.storagePath }}
+          {{- else }}
             - --storage.tsdb.path={{ .Values.server.persistentVolume.mountPath }}
+          {{- end }}
             - --web.console.libraries=/etc/prometheus/console_libraries
             - --web.console.templates=/etc/prometheus/consoles
           {{- range .Values.server.extraFlags }}