User-Agent "Python-gRPC-" → "grpc-python/"

This brings gRPC Python into conformance with the recommendation in
https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md.

src/python/grpcio/grpc/_channel.py

@@ -39,7 +39,7 @@ from grpc import _grpcio_metadata
 from grpc._cython import cygrpc
 from grpc.framework.foundation import callable_util
 
-_USER_AGENT = 'Python-gRPC-{}'.format(_grpcio_metadata.__version__)
+_USER_AGENT = 'grpc-python/{}'.format(_grpcio_metadata.__version__)
 
 _EMPTY_FLAGS = 0
 _INFINITE_FUTURE = cygrpc.Timespec(float('+inf'))

src/python/grpcio_tests/tests/unit/_metadata_test.py

@@ -32,7 +32,7 @@ import unittest
 import weakref
 
 import grpc
-from grpc import _grpcio_metadata
+from grpc import _channel
 from grpc.framework.foundation import logging_pool
 
 from tests.unit import test_common
@@ -49,8 +49,6 @@ _UNARY_STREAM = '/test/UnaryStream'
 _STREAM_UNARY = '/test/StreamUnary'
 _STREAM_STREAM = '/test/StreamStream'
 
-_USER_AGENT = 'Python-gRPC-{}'.format(_grpcio_metadata.__version__)
-
 _CLIENT_METADATA = (('client-md-key', 'client-md-key'),
                     ('client-md-key-bin', b'\x00\x01'))
 
@@ -76,7 +74,7 @@ def validate_client_metadata(test, servicer_context):
             _CLIENT_METADATA, servicer_context.invocation_metadata()))
     test.assertTrue(
         user_agent(servicer_context.invocation_metadata())
-        .startswith('primary-agent ' + _USER_AGENT))
+        .startswith('primary-agent ' + _channel._USER_AGENT))
     test.assertTrue(
         user_agent(servicer_context.invocation_metadata())
         .endswith('secondary-agent'))