Make sure getOpsMetrics$ only replays lastest value (#77673)

Author: joshdover

src/core/server/metrics/metrics_service.test.ts

@@ -106,6 +106,25 @@ describe('MetricsService', () => {
         `"#setup() needs to be run first"`
       );
     });
+
+    it('emits the last value on each getOpsMetrics$ call', async () => {
+      const firstMetrics = { metric: 'first' };
+      const secondMetrics = { metric: 'second' };
+      mockOpsCollector.collect
+        .mockResolvedValueOnce(firstMetrics)
+        .mockResolvedValueOnce(secondMetrics);
+
+      await metricsService.setup({ http: httpMock });
+      const { getOpsMetrics$ } = await metricsService.start();
+
+      const firstEmission = getOpsMetrics$().pipe(take(1)).toPromise();
+      jest.advanceTimersByTime(testInterval);
+      expect(await firstEmission).toEqual({ metric: 'first' });
+
+      const secondEmission = getOpsMetrics$().pipe(take(1)).toPromise();
+      jest.advanceTimersByTime(testInterval);
+      expect(await secondEmission).toEqual({ metric: 'second' });
+    });
   });
 
   describe('#stop', () => {

src/core/server/metrics/metrics_service.ts

@@ -37,7 +37,7 @@ export class MetricsService
   private readonly logger: Logger;
   private metricsCollector?: OpsMetricsCollector;
   private collectInterval?: NodeJS.Timeout;
-  private metrics$ = new ReplaySubject<OpsMetrics>();
+  private metrics$ = new ReplaySubject<OpsMetrics>(1);
   private service?: InternalMetricsServiceSetup;
 
   constructor(private readonly coreContext: CoreContext) {