Discover only databases that are not templates and allow connections (prometheus-community#297)

* Discover only databases that are not templates and allow connections

* Update readme

Author: alexissellier

README.md

@@ -163,7 +163,7 @@ flag. This removes all built-in metrics, and uses only metrics defined by querie
 
 ### Automatically discover databases
 To scrape metrics from all databases on a database server, the database DSN's can be dynamically discovered via the 
-`--auto-discover-databases` flag. When true, `SELECT datname FROM pg_database` is run for all configured DSN's. From the 
+`--auto-discover-databases` flag. When true, `SELECT datname FROM pg_database WHERE datallowconn = true AND datistemplate = false` is run for all configured DSN's. From the 
 result a new set of DSN's is created for which the metrics are scraped.
 
 In addition, the option `--exclude-databases` adds the possibily to filter the result from the auto discovery to discard databases you do not need.

cmd/postgres_exporter/postgres_exporter.go

@@ -1074,7 +1074,7 @@ func newDesc(subsystem, name, help string, labels prometheus.Labels) *prometheus
 }
 
 func queryDatabases(server *Server) ([]string, error) {
-	rows, err := server.db.Query("SELECT datname FROM pg_database") // nolint: safesql
+	rows, err := server.db.Query("SELECT datname FROM pg_database WHERE datallowconn = true AND datistemplate = false") // nolint: safesql
 	if err != nil {
 		return nil, fmt.Errorf("Error retrieving databases: %v", err)
 	}