Prometheus: Add off switch for metric/label name lookup (#24034)

* Prometheus: Add off switch for metric/label name lookup

This will help users with amounts of metric name
data that is too much for a browser to handle.

Autocomplete will be disabled and metrics chooser hidden,
since obviously both rely on this data.

Fixes #22702

* Use onUpdateDatasourceJsonDataOptionChecked

... from `@grafana/data`. Refactor naming to faciliate its
use and stick with prop names as passed down from
`ConfigEditor`.

PLUS:
- Rephrase switch label, add a tooltip and reduce the
size of the to what "Custom query parameters" originally
was.
- Change `languageProvider` type in `PromQueryField`.

* Put language provider back in

Functions and history still work, even when metrics
lookup gets disabled.
Also: Rewording of setting.

* Display a message when lookup got disabled manually

* Call property for setting disableMetricsLookup

* Show disabled metrics chooser instead of warning

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

@@ -112,10 +112,15 @@ export default class PromQlLanguageProvider extends LanguageProvider {
   };
 
   start = async (): Promise<any[]> => {
+    if (this.datasource.lookupsDisabled) {
+      return [];
+    }
+
     this.metrics = await this.request('/api/v1/label/__name__/values', []);
     this.lookupsDisabled = this.metrics.length > this.lookupMetricsThreshold;
     this.metricsMetadata = await this.request('/api/v1/metadata', {});
     this.processHistogramMetrics(this.metrics);
+
     return [];
   };