Backoff to avoid excessive retries to Run Service in a duration (#3354)

src/Test/L0/TestHostContext.cs

@@ -30,9 +30,11 @@ namespace GitHub.Runner.Common.Tests
         private string _tempDirectoryRoot = Path.Combine(Path.GetTempPath(), Guid.NewGuid().ToString("D"));
         private StartupType _startupType;
         public event EventHandler Unloading;
+        public event EventHandler<DelayEventArgs> Delaying;
         public CancellationToken RunnerShutdownToken => _runnerShutdownTokenSource.Token;
         public ShutdownReason RunnerShutdownReason { get; private set; }
         public ISecretMasker SecretMasker => _secretMasker;
+
         public TestHostContext(object testClass, [CallerMemberName] string testName = "")
         {
             ArgUtil.NotNull(testClass, nameof(testClass));
@@ -92,6 +94,14 @@ namespace GitHub.Runner.Common.Tests
 
         public async Task Delay(TimeSpan delay, CancellationToken token)
         {
+            // Event callback
+            EventHandler<DelayEventArgs> handler = Delaying;
+            if (handler != null)
+            {
+                handler(this, new DelayEventArgs(delay, token));
+            }
+
+            // Delay zero
             await Task.Delay(TimeSpan.Zero);
         }
 
@@ -361,4 +371,19 @@ namespace GitHub.Runner.Common.Tests
             }
         }
     }
+
+    public class DelayEventArgs : EventArgs
+    {
+        public DelayEventArgs(
+            TimeSpan delay,
+            CancellationToken token)
+        {
+            Delay = delay;
+            Token = token;
+        }
+
+        public TimeSpan Delay { get; }
+
+        public CancellationToken Token { get; }
+    }
 }