Fix grpc tests when running from cmd-line/eachdist script (#1027)

* when running the grpc tests with pytest or eachdist from the command
  line the 2nd test trying to connect to the test server failed with
  a connection refused message.
  Seems like the connection from the previous test was still alive due to
  the channel not being properly closed.

Author: mariojonke

Diff for: instrumentation/opentelemetry-instrumentation-grpc/setup.cfg

@@ -42,7 +42,7 @@ packages=find_namespace:
 install_requires =
     opentelemetry-api == 0.13dev0
     opentelemetry-sdk == 0.13dev0
-    grpcio == 1.30
+    grpcio ~= 1.27
 
 [options.extras_require]
 test =

Diff for: instrumentation/opentelemetry-instrumentation-grpc/tests/test_client_interceptor.py

@@ -49,6 +49,7 @@ def tearDown(self):
         GrpcInstrumentorClient().uninstrument()
         self.memory_metrics_exporter.clear()
         self.server.stop(None)
+        self.channel.close()
 
     def _verify_success_records(self, num_bytes_out, num_bytes_in, method):
         # pylint: disable=protected-access,no-member