Explore: async starts of language provider

- changed `start()` to return promise on main language feature task
- promise resolves to list of secondary tasks
- speeds up time to interaction of metric selector
- lazy loading of certain metric selector and log label selector items
- loading indication of metric and log label selectors

public/app/plugins/datasource/prometheus/language_provider.ts

@@ -74,7 +74,7 @@ export default class PromQlLanguageProvider extends LanguageProvider {
   start = () => {
     if (!this.started) {
       this.started = true;
-      return Promise.all([this.fetchMetricNames(), this.fetchHistogramMetrics()]);
+      return this.fetchMetricNames().then(() => [this.fetchHistogramMetrics()]);
     }
     return Promise.resolve([]);
   };