Add activity source to allow OpenTelemetry to collect tracing (#645)

* add activity source

* add minimum requirement for MinBackOffDuration

* Revert "add minimum requirement for MinBackOffDuration"

This reverts commit 59d847a.

* revert make MinBackoffDuration public

* update

* update activity name

* update

Author: zhiyuanliang-ms

src/Microsoft.Extensions.Configuration.AzureAppConfiguration/AzureAppConfigurationProvider.cs

@@ -23,6 +23,7 @@ namespace Microsoft.Extensions.Configuration.AzureAppConfiguration
 {
     internal class AzureAppConfigurationProvider : ConfigurationProvider, IConfigurationRefresher, IDisposable
     {
+        private readonly ActivitySource _activitySource = new ActivitySource(ActivityNames.AzureAppConfigurationActivitySource);
         private bool _optional;
         private bool _isInitialLoadComplete = false;
         private bool _isAssemblyInspected;
@@ -158,7 +159,7 @@ public AzureAppConfigurationProvider(IConfigurationClientManager configClientMan
         public override void Load()
         {
             var watch = Stopwatch.StartNew();
-
+            using Activity activity = _activitySource.StartActivity(ActivityNames.Load);
             try
             {
                 using var startupCancellationTokenSource = new CancellationTokenSource(_options.Startup.Timeout);
@@ -258,6 +259,7 @@ public async Task RefreshAsync(CancellationToken cancellationToken)
                         return;
                     }
 
+                    using Activity activity = _activitySource.StartActivity(ActivityNames.Refresh);
                     // Check if initial configuration load had failed
                     if (_mappedData == null)
                     {
@@ -1406,6 +1408,7 @@ private async Task ProcessKeyValueChangesAsync(
         public void Dispose()
         {
             (_configClientManager as ConfigurationClientManager)?.Dispose();
+            _activitySource.Dispose();
         }
     }
 }

src/Microsoft.Extensions.Configuration.AzureAppConfiguration/Constants/ActivityNames.cs

@@ -0,0 +1,13 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+//
+
+namespace Microsoft.Extensions.Configuration.AzureAppConfiguration
+{
+    internal static class ActivityNames
+    {
+        public const string AzureAppConfigurationActivitySource = "Microsoft.Extensions.Configuration.AzureAppConfiguration";
+        public const string Load = "Load";
+        public const string Refresh = "Refresh";
+    }
+}