Prometheus: Display HELP and TYPE of metrics if available (#21124)

* Prometheus: Display HELP and TYPE of metrics if available

- Prometheus recently added a metadata API around HELP and TYPE of
metrics
- request metadata when datasource instance is created
- use metadata to show help and type in typeahead suggestions and in
metrics selector as tooltip

* Fix types

packages/grafana-ui/src/components/Cascader/Cascader.tsx

@@ -9,6 +9,8 @@ export interface CascaderOption {
 
   children?: CascaderOption[];
   disabled?: boolean;
+  // Undocumented tooltip API
+  title?: string;
 }
 
 export interface CascaderProps {