title: Read gRPC metadata in Python server
body: <p>I have client like this:</p>
  
  <pre><code>def run():
      channel = grpc.insecure_channel('localhost:50051')
      stub = helloworld_pb2_grpc.GreeterStub(channel)
      metadata = [('key', "value")]
      response = stub.SayHello(
          helloworld_pb2.HelloRequest(name='you'),
          metadata=metadata
      )
      print("Greeter client received: " + response.message)
  </code></pre>
  
  <p>And server:</p>
  
  <pre><code>class Greeter(helloworld_pb2_grpc.GreeterServicer):    
      def SayHello(self, request, context):
          return helloworld_pb2.HelloReply(message='Hello, %s!' % request.name)
  </code></pre>
  
  <p>So, in server, how can I read metadata sent by client?</p>