Switching license polling from a switchMap to an exhaustMap

When it's ES is slow to respond with a license, or Kibana is overloaded
and is slow making the request or handling the response, we were trying
to fetch the license again. This will just skip that refresh interval,
and catch it the next time around.

Author: kobelb

x-pack/plugins/licensing/common/license_update.ts

@@ -5,7 +5,7 @@
  */
 import { ConnectableObservable, Observable, Subject, from, merge } from 'rxjs';
 
-import { filter, map, pairwise, switchMap, publishReplay, takeUntil } from 'rxjs/operators';
+import { filter, map, pairwise, exhaustMap, publishReplay, takeUntil } from 'rxjs/operators';
 import { hasLicenseInfoChanged } from './has_license_info_changed';
 import { ILicense } from './types';
 
@@ -15,7 +15,7 @@ export function createLicenseUpdate(
   fetcher: () => Promise<ILicense>,
   initialValues?: ILicense
 ) {
-  const triggerRefresh$ = trigger$.pipe(switchMap(fetcher));
+  const triggerRefresh$ = trigger$.pipe(exhaustMap(fetcher));
   const manuallyFetched$ = new Subject<ILicense>();
 
   const fetched$ = merge(triggerRefresh$, manuallyFetched$).pipe(