feat: Allow ActivitySource to be specified in ServiceSettingsBase

While we *could* remove ActivitySource from ClientHelper.Options, that would require the settings to be cloned if the user hasn't specified an activity source. That could potentially be done in ClientBuilderBase, but keeping the default separate from the user-specified activity source feels simpler.

Google.Api.Gax.Grpc/ClientHelper.cs

@@ -64,7 +64,7 @@ public ClientHelper(Options options)
             {
                 _versionCallSettings = _versionCallSettings.WithHeader(ApiVersionHeaderName, options.ApiVersion);
             }
-            _activitySource = options.ActivitySource;
+            _activitySource = settings.ActivitySource ?? options.ActivitySource;
         }
 
         /// <summary>
@@ -214,7 +214,8 @@ public sealed class Options
             public string ApiVersion { get; set; }
 
             /// <summary>
-            /// The activity source to use for tracing, if any. This may be null.
+            /// The activity source to use for tracing, if any. This may be null. This is ignored
+            /// if <see cref="Settings"/> specifies an activity source.
             /// Note: currently internal until we're ready to roll out OpenTelemetry support "properly".
             /// </summary>
             internal ActivitySource ActivitySource { get; set; }

Google.Api.Gax.Grpc/ServiceSettingsBase.cs

@@ -7,6 +7,7 @@
 
 using Grpc.Core;
 using Grpc.Core.Interceptors;
+using System.Diagnostics;
 
 namespace Google.Api.Gax.Grpc
 {
@@ -42,6 +43,7 @@ protected ServiceSettingsBase(ServiceSettingsBase existing)
             Scheduler = existing.Scheduler;
             VersionHeaderBuilder = existing.VersionHeaderBuilder.Clone();
             Interceptor = existing.Interceptor;
+            ActivitySource = existing.ActivitySource;
         }
 
         /// <summary>
@@ -84,5 +86,12 @@ protected ServiceSettingsBase(ServiceSettingsBase existing)
         /// on or after that date are aware of this property.
         /// </summary>
         public Interceptor Interceptor { get; set; }
+
+        /// <summary>
+        /// An optional <see cref="ActivitySource"/>, which can override the default activity source used for tracing
+        /// calls from this client.
+        /// Note: currently internal until we're ready to roll out OpenTelemetry support "properly".
+        /// </summary>
+        internal ActivitySource ActivitySource { get; set; }
     }
 }