Prometheus: Show "Loading metrics..." when switching sources (#24292)

syntaxLoaded needed resetting at the beginning of refreshing
metrics.

Fixes #24291

(cherry picked from commit dfe2ab9502)

public/app/plugins/datasource/prometheus/components/PromQueryField.tsx

@@ -187,6 +187,10 @@ class PromQueryField extends React.PureComponent<PromQueryFieldProps, PromQueryF
       datasource: { languageProvider },
     } = this.props;
 
+    this.setState({
+      syntaxLoaded: false,
+    });
+
     Prism.languages[PRISM_SYNTAX] = languageProvider.syntax;
     this.languageProviderInitializationPromise = makePromiseCancelable(languageProvider.start());
     this.languageProviderInitializationPromise.promise