Rolling average work queue diagnostic (#24933).

pkg/analysis_server/test/domain_diagnostic_test.dart

Index: pkg/analysis_server/test/domain_diagnostic_test.dart
diff --git a/pkg/analysis_server/test/domain_diagnostic_test.dart b/pkg/analysis_server/test/domain_diagnostic_test.dart
index d13f2af1077ef37b75af3cceb581f9a39516d855..f6b4e5353ec2b6ea1112b3d4429c2d3de600bb88 100644
--- a/pkg/analysis_server/test/domain_diagnostic_test.dart
+++ b/pkg/analysis_server/test/domain_diagnostic_test.dart
@@ -46,6 +46,10 @@ main() {
     handler = new DiagnosticDomainHandler(server);
   });
 
+  tearDown(() {
+    handler.sampler?.stop();
+  });
+
   group('DiagnosticDomainHandler', () {
     test('getDiagnostics', () async {
       String file = '/project/bin/test.dart';
@@ -68,6 +72,7 @@ main() {
       expect(context['explicitFileCount'], fileCount);
       expect(context['implicitFileCount'], 0);
       expect(context['workItemQueueLength'], isNotNull);
+      expect(context['workItemQueueLengthAverage'], isNotNull);
     });
 
     test('getDiagnostics - (no root)', () async {