Fix race condition causing flaky PeriodicMetricReaderTest (#8317)

Author: jack-berg

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

@@ -214,17 +214,17 @@ CompletableResultCode doRun() {
           }
           if (metricData.isEmpty()) {
             logger.log(Level.FINE, "No metric data to export - skipping export.");
-            flushResult.succeed();
             exportAvailable.set(true);
+            flushResult.succeed();
           } else {
             CompletableResultCode result = exporter.export(metricData);
             result.whenComplete(
                 () -> {
                   if (!result.isSuccess()) {
                     logger.log(Level.FINE, "Exporter failed");
                   }
-                  flushResult.succeed();
                   exportAvailable.set(true);
+                  flushResult.succeed();
                 });
           }
         } catch (Throwable t) {