Propagate flush to PeriodicMetricReader's metricExporter.

On a synchronous exporter this doesn't matter, but the API design allows for asynchronous exporters, which requires calling flush after manually exporting.

Fixes #7343

Author: tylerbenson

sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/export/PeriodicMetricReader.java

@@ -81,7 +81,21 @@ public MemoryMode getMemoryMode() {
 
   @Override
   public CompletableResultCode forceFlush() {
-    return scheduled.doRun();
+    CompletableResultCode result = new CompletableResultCode();
+    CompletableResultCode flushResult = scheduled.doRun();
+    flushResult.whenComplete(
+        () ->
+            exporter
+                .flush()
+                .whenComplete(
+                    () -> {
+                      if (!flushResult.isSuccess()) {
+                        result.fail();
+                      } else {
+                        result.succeed();
+                      }
+                    }));
+    return result;
   }
 
   @Override

sdk/metrics/src/test/java/io/opentelemetry/sdk/metrics/export/PeriodicMetricReaderTest.java

@@ -154,6 +154,28 @@ void flush() throws Exception {
     }
   }
 
+  @Test
+  void forceflush_callsFlush() {
+    MetricExporter metricExporter = mock(MetricExporter.class);
+    when(metricExporter.export(any())).thenReturn(CompletableResultCode.ofSuccess());
+    when(metricExporter.flush()).thenReturn(CompletableResultCode.ofSuccess());
+    when(metricExporter.shutdown()).thenReturn(CompletableResultCode.ofSuccess());
+
+    PeriodicMetricReader reader =
+        PeriodicMetricReader.builder(metricExporter)
+            .setInterval(Duration.ofNanos(Long.MAX_VALUE))
+            .build();
+
+    reader.register(collectionRegistration);
+    assertThat(reader.forceFlush().join(10, TimeUnit.SECONDS).isSuccess()).isTrue();
+
+    try {
+      verify(metricExporter).flush();
+    } finally {
+      reader.shutdown();
+    }
+  }
+
   @Test
   @Timeout(2)
   @SuppressLogger(PeriodicMetricReader.class)