[7.8] Fix flaky ServerMetricsCollector integration test (elastic#65420)…

… (elastic#66643)

src/core/server/metrics/integration_tests/server_collector.test.ts

@@ -185,18 +185,22 @@ describe('ServerMetricsCollector', () => {
     let metrics = await collector.collect();
     expect(metrics.concurrent_connections).toEqual(0);
 
-    sendGet('/').end(() => null);
+    // supertest requests are executed when calling `.then` (or awaiting them).
+    // however in this test we need to send the request now and await for it later in the code.
+    // also using `.end` is not possible as it would execute the request twice.
+    // so the only option is this noop `.then`.
+    const res1 = sendGet('/').then(res => res);
     await waitForHits(1);
     metrics = await collector.collect();
     expect(metrics.concurrent_connections).toEqual(1);
 
-    sendGet('/').end(() => null);
+    const res2 = sendGet('/').then(res => res);
     await waitForHits(2);
     metrics = await collector.collect();
     expect(metrics.concurrent_connections).toEqual(2);
 
     waitSubject.next('go');
-    await delay(requestWaitDelay);
+    await Promise.all([res1, res2]);
     metrics = await collector.collect();
     expect(metrics.concurrent_connections).toEqual(0);
   });