chore: Add comments for setting the sampledLocalTracing flag

Author: lqiu96

gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/grpc/GrpcServiceStubClassComposer.java

@@ -141,6 +141,8 @@ protected Statement createMethodDescriptorVariableDecl(
             .apply("setResponseMarshaller", protoUtilsMarshallerFn.apply(methodInvocationArg))
             .apply(methodDescriptorMaker);
 
+    // The sampledToLocalTracing flag is set to true for all gRPC MethodDescriptors in GAPICs
+    // This flag enables captures for specific method names to help provide more detailed metrics
     methodDescriptorMaker =
         methodMakerFn
             .apply(