fix(postgres-system): fix unexpected behaviour difference

This patch fixes the unexpected change in bahaviour. When using a
namespace selector that selects one or multiple namespaces, there will
be two labels. `namespace`, where the podMonitor lives, and
`exported_namespace`, where the target pods is. When using a podMonitor
that scans any namespace, there is only a `namespace` label.

infrastructure/postgres/dashboards/postgresql-database.json

@@ -3032,7 +3032,7 @@
         "options": [],
         "query": "query_result(pg_exporter_last_scrape_duration_seconds)",
         "refresh": 2,
-        "regex": "/.*exported_namespace=\"([^\"]+).*/",
+        "regex": "/.*namespace=\"([^\"]+).*/",
         "skipUrlSync": false,
         "sort": 1,
         "tagValuesQuery": "",
@@ -3052,7 +3052,7 @@
         "multi": false,
         "name": "release",
         "options": [],
-        "query": "query_result(pg_exporter_last_scrape_duration_seconds{exported_namespace=\"$namespace\"})",
+        "query": "query_result(pg_exporter_last_scrape_duration_seconds{namespace=\"$namespace\"})",
         "refresh": 2,
         "regex": "/.*cluster_name=\"([^\"]+)/",
         "skipUrlSync": false,