Add support for DATA_SOURCE_NAME_FILE

Signed-off-by: Björn Wenzel <bjoern.wenzel@dbschenker.com>

README.md

@@ -93,6 +93,9 @@ The following environment variables configure the exporter:
 * `DATA_SOURCE_NAME`
   the default legacy format. Accepts URI form and key=value form arguments. The
   URI may contain the username and password to connect with.
+  
+* `DATA_SOURCE_NAME_FILE`
+  The same as above but reads the URI from a file.
 
 * `DATA_SOURCE_URI`
    an alternative to `DATA_SOURCE_NAME` which exclusively accepts the hostname

cmd/postgres_exporter/datasource.go

@@ -126,6 +126,16 @@ func getDataSources() ([]string, error) {
 		return strings.Split(dsn, ","), nil
 	}
 
+	dataSourceNameFile := os.Getenv("DATA_SOURCE_NAME_FILE")
+	if len(dataSourceNameFile) != 0 {
+		fileContents, err := ioutil.ReadFile(dataSourceNameFile)
+		if err != nil {
+			return nil, fmt.Errorf("failed loading data source name file %s: %s", dataSourceNameFile, err.Error())
+		}
+		dsnContent := strings.TrimSpace(string(fileContents))
+		return strings.Split(dsnContent, ","), nil
+	}
+
 	var user, pass, uri string
 
 	dataSourceUserFile := os.Getenv("DATA_SOURCE_USER_FILE")