Mark channelArg as copy

src/objective-c/GRPCClient/private/GRPCChannel.m

@@ -265,7 +265,7 @@ static GRPCChannelPool *gChannelPool;
     [args addEntriesFromDictionary:config.callOptions.additionalChannelArgs];
     channelArgs = args;
   } else {
-    channelArgs = [config.channelArgs copy];
+    channelArgs = config.channelArgs;
   }
   id<GRPCChannelFactory> factory = config.channelFactory;
   grpc_channel *unmanaged_channel = [factory createChannelWithHost:host channelArgs:channelArgs];

src/objective-c/GRPCClient/private/GRPCChannelPool.h

@@ -45,7 +45,7 @@ NS_ASSUME_NONNULL_BEGIN
 @property(readonly) id<GRPCChannelFactory> channelFactory;
 
 /** Acquire the dictionary of channel args with current configurations. */
-@property(readonly) NSDictionary *channelArgs;
+@property(copy, readonly) NSDictionary *channelArgs;
 
 - (nullable instancetype)initWithHost:(NSString *)host callOptions:(GRPCCallOptions *)callOptions;