Log new event when CredentialType is selected for DefaultAzureCredential (Azure#18906)

* logged more specific event for credential selection

Author: christothes

sdk/identity/Azure.Identity/src/AzureIdentityEventSource.cs

@@ -27,6 +27,7 @@ internal sealed class AzureIdentityEventSource : AzureEventSource
         private const int MsalLogErrorEvent = 10;
         private const int InteractiveAuthenticationThreadPoolExecutionEvent = 11;
         private const int InteractiveAuthenticationInlineExecutionEvent = 12;
+        private const int DefaultAzureCredentialCredentialSelectedEvent = 13;
 
         private AzureIdentityEventSource() : base(EventSourceName) { }
 
@@ -176,8 +177,6 @@ public void LogMsal(Microsoft.Identity.Client.LogLevel level, string message, bo
                     case Microsoft.Identity.Client.LogLevel.Verbose when IsEnabled(EventLevel.Verbose, EventKeywords.All):
                         LogMsalVerbose(message);
                         break;
-                    default:
-                        break;
                 }
             }
         }
@@ -223,5 +222,11 @@ public void InteractiveAuthenticationExecutingInline()
         {
             WriteEvent(InteractiveAuthenticationInlineExecutionEvent);
         }
+
+        [Event(DefaultAzureCredentialCredentialSelectedEvent, Level = EventLevel.Informational, Message = "DefaultAzureCredential credential selected: {0}")]
+        public void DefaultAzureCredentialCredentialSelected(string credentialType)
+        {
+            WriteEvent(DefaultAzureCredentialCredentialSelectedEvent, credentialType);
+        }
     }
 }

sdk/identity/Azure.Identity/src/DefaultAzureCredential.cs

@@ -134,13 +134,14 @@ private async ValueTask<AccessToken> GetTokenImplAsync(bool async, TokenRequestC
                     (token, credential) = await GetTokenFromSourcesAsync(_sources, requestContext, async, cancellationToken).ConfigureAwait(false);
                     _sources = default;
                     asyncLock.SetValue(credential);
+                    AzureIdentityEventSource.Singleton.DefaultAzureCredentialCredentialSelected(credential.GetType().FullName);
                 }
 
                 return scope.Succeeded(token);
             }
             catch (Exception e)
             {
-               throw scope.FailWrapAndThrow(e);
+                throw scope.FailWrapAndThrow(e);
             }
         }