feat(mixin): Add grafana mixins support for postgres exporter

Signed-off-by: Kaviraj <kavirajkanagaraj@gmail.com>

postgres_mixin/mixin.libsonnet

@@ -0,0 +1,13 @@
+{
+  grafanaDashboards: {
+    'postgres-overview.json': (import 'dashboards/postgres-overview.json'),
+  },
+
+  // Helper function to ensure that we don't override other rules, by forcing
+  // the patching of the groups list, and not the overall rules object.
+  local importRules(rules) = {
+    groups+: std.native('parseYaml')(rules)[0].groups,
+  },
+
+  prometheusAlerts+: importRules(importstr 'alerts/alerts.yaml'),
+}