Fix queries for awsrds (#370)

* excluding rolname 'rdsadmin' in pg_stat_statements

* notes on using postgres-exporter with AWS:RDS

README-RDS.md

@@ -0,0 +1,38 @@
+# Using Postgres-Exporter with AWS:RDS
+
+### When using postgres-exporter with Amazon Web Services' RDS, the
+  rolname "rdsadmin" and datname "rdsadmin" must be excluded.
+
+I had success running docker container 'wrouesnel/postgres_exporter:latest'
+with queries.yaml as the PG_EXPORTER_EXTEND_QUERY_PATH.  errors
+mentioned in issue#335 appeared and I had to modify the
+'pg_stat_statements' query with the following:
+`WHERE t2.rolname != 'rdsadmin'`
+
+Running postgres-exporter in a container like so:
+  ```
+  DBNAME='postgres'
+  PGUSER='postgres'
+  PGPASS='psqlpasswd123'
+  PGHOST='name.blahblah.us-east-1.rds.amazonaws.com'
+  docker run --rm --detach \
+      --name "postgresql_exporter_rds" \
+      --publish 9187:9187 \
+      --volume=/etc/prometheus/postgresql-exporter/queries.yaml:/var/lib/postgresql/queries.yaml \
+      -e DATA_SOURCE_NAME="postgresql://${PGUSER}:${PGPASS}@${PGHOST}:5432/${DBNAME}?sslmode=disable" \
+      -e PG_EXPORTER_EXCLUDE_DATABASES=rdsadmin \
+      -e PG_EXPORTER_DISABLE_DEFAULT_METRICS=true \
+      -e PG_EXPORTER_DISABLE_SETTINGS_METRICS=true \
+      -e PG_EXPORTER_EXTEND_QUERY_PATH='/var/lib/postgresql/queries.yaml' \
+      wrouesnel/postgres_exporter
+  ```
+
+### Expected changes to RDS:
++ see stackoverflow notes
+  (https://stackoverflow.com/questions/43926499/amazon-postgres-rds-pg-stat-statements-not-loaded#43931885)
++ you must also use a specific RDS parameter_group that includes the following:
+  ```
+  shared_preload_libraries = "pg_stat_statements,pg_hint_plan"
+  ```
++ lastly, you must reboot the RDS instance.
+