Update GrpcClient to client ctor on channel (#15448)

* Update GrpcClient to client ctor on channel

Fixes #14136

* Remove duplicated code sections

Author: jongalloway

aspnetcore/tutorials/grpc/readme.md

@@ -141,10 +141,9 @@ namespace GrpcGreeterClient
     {
         static async Task Main(string[] args)
         {
-            var httpClient = new HttpClient();
             // The port number(5001) must match the port of the gRPC server.
-            httpClient.BaseAddress = new Uri("https://localhost:5001");
-            var client = GrpcClient.Create<Greeter.GreeterClient>(httpClient);
+            var channel = GrpcChannel.ForAddress("https://localhost:5001");
+            var client = new Greeter.GreeterClient(channel);
             var reply = await client.SayHelloAsync(
                               new HelloRequest { Name = "GreeterClient" });
             Console.WriteLine("Greeting: " + reply.Message);
@@ -159,40 +158,8 @@ namespace GrpcGreeterClient
 
 The Greeter client is created by:
 
-* Instantiating an `HttpClient` containing the information for creating the connection to the gRPC service.
-* Using the `HttpClient` to construct the Greeter client:
-
-```csharp
-static async Task Main(string[] args)
-{
-    var httpClient = new HttpClient();
-    // The port number(5001) must match the port of the gRPC server.
-    httpClient.BaseAddress = new Uri("https://localhost:5001");
-    var client = GrpcClient.Create<Greeter.GreeterClient>(httpClient);
-    var reply = await client.SayHelloAsync(
-                      new HelloRequest { Name = "GreeterClient" });
-    Console.WriteLine("Greeting: " + reply.Message);
-    Console.WriteLine("Press any key to exit...");
-    Console.ReadKey();
-}
-```
-
-The Greeter client calls the asynchronous `SayHello` method. The result of the `SayHello` call is displayed:
-
-```csharp
-static async Task Main(string[] args)
-{
-    var httpClient = new HttpClient();
-    // The port number(5001) must match the port of the gRPC server.
-    httpClient.BaseAddress = new Uri("https://localhost:5001");
-    var client = GrpcClient.Create<Greeter.GreeterClient>(httpClient);
-    var reply = await client.SayHelloAsync(
-                      new HelloRequest { Name = "GreeterClient" });
-    Console.WriteLine("Greeting: " + reply.Message);
-    Console.WriteLine("Press any key to exit...");
-    Console.ReadKey();
-}
-```
+* Instantiating a `GrpcChannel` containing the information for creating the connection to the gRPC service.
+* Using the `GrpcChannel` to construct the Greeter client.
 
 ## Test the gRPC client with the gRPC Greeter service