[-] use proper datasource id in Prometheus dashboards (#972)

Author: pashagolub

grafana/postgres_datasource.yml

@@ -1,7 +1,7 @@
 apiVersion: 1
 
 datasources:
-- name: pgwatch metrics
+- name: pgwatch metrics (postgres)
   uid: pgwatch-metrics
   type: postgres
   url: postgres:5432