[stable/postgresql] Fix metrics volume arguments (#7987)

"-extend.query-path" is invalid argument to postgres_exporter since
0.4.x version due to cli library update (ref:
https://github.com/wrouesnel/postgres_exporter/issues/147)

Currently, as a result of invalid argument, metrics container fails
to start.

This commit fixes the issue by replacing broken command line argument
with the corresponding environment variable, as they are more consistent.

Signed-off-by: Igor Zibarev <zibarev.i@gmail.com>

stable/postgresql/Chart.yaml

@@ -1,5 +1,5 @@
 name: postgresql
-version: 0.18.1
+version: 0.18.2
 appVersion: 9.6.2
 description: Object-relational database management system (ORDBMS) with an emphasis on extensibility and on standards-compliance.
 keywords:

stable/postgresql/templates/deployment.yaml

@@ -128,11 +128,14 @@ spec:
         env:
         - name: DATA_SOURCE_NAME
           value: postgresql://{{ default "postgres" .Values.postgresUser }}@127.0.0.1:5432?sslmode=disable
+        {{- if .Values.metrics.customMetrics }}
+        - name: PG_EXPORTER_EXTEND_QUERY_PATH
+          value: /conf/custom-metrics.yaml
+        {{- end }}
         ports:
         - name: metrics
           containerPort: 9187
         {{- if .Values.metrics.customMetrics }}
-        args: ["-extend.query-path", "/conf/custom-metrics.yaml"]
         volumeMounts:
           - name: custom-metrics
             mountPath: /conf