Prometheus: Metrics browser (#33847)

* [WIP] Metrics browser

* Removed unused import

* Metrics selection logic

* Remove redundant tests

All data is fetched now regardless to the current range so test for checking reloading the data on the range change are no longer relevant.

* Remove commented out code blocks

* Add issue number to todos

Co-authored-by: Piotr Jamróz <pm.jamroz@gmail.com>

public/app/plugins/datasource/prometheus/components/PromExploreQueryEditor.test.tsx

@@ -10,6 +10,8 @@ const setup = (renderMethod: any, propOverrides?: object) => {
   const datasourceMock: unknown = {
     languageProvider: {
       syntax: () => {},
+      getLabelKeys: () => [],
+      metrics: [],
     },
   };
   const datasource: PrometheusDatasource = datasourceMock as PrometheusDatasource;