gRPC instrumentor server does not work

[cnnradams]

Describe your environment
python3.7, working out of #878

Steps to reproduce
I have a sample gRPC server/client in #878, so checkout that and then run the test

import grpc

from opentelemetry.ext.grpc import server_interceptor
from opentelemetry.ext.grpc.grpcext import intercept_server
from opentelemetry.test.test_base import TestBase

from ._client import simple_method, server_streaming_method, client_streaming_method, bidirectional_streaming_method
from ._server import create_test_server
from tests.protobuf import test_server_pb2_grpc

class TestServerProto(TestBase):
    def setUp(self):
        super().setUp()
        server = create_test_server(25565)

        interceptor = server_interceptor()
        self.server = intercept_server(server, interceptor)

        self.server.start()

        self.channel = grpc.insecure_channel("localhost:25565")
        self._stub = test_server_pb2_grpc.GRPCTestServerStub(self.channel)

    def tearDown(self):
        super().tearDown()
        self.server.stop(None)

    def test_unary_unary(self):
        simple_method(self._stub)

        spans = self.memory_exporter.get_finished_spans()
        self.assertEqual(len(spans), 1)
        span = spans[0]

It raises

______________________________________________________________________________ TestServerProto.test_unary_unary _______________________________________________________________________________

self = <tests.test_server_interceptor.TestServerProto testMethod=test_unary_unary>

    def test_unary_unary(self):
        simple_method(self._stub)
    
        spans = self.memory_exporter.get_finished_spans()
>       self.assertEqual(len(spans), 1)
E       AssertionError: 0 != 1

test_server_interceptor.py:32: AssertionError

What is the expected behavior?
There should be a span since I ran intercept_server, as shown in test_server_interceptor.py

Additional context
test_server_interceptor.py works, what is the difference between my code example and it (does my auto-generated pb2 not actually call channel.unary_unary)?

The actual example works properly (excluding server tracing of course), and can be instrumented by the client, see test_client_interceptor.py

also note that my PR did not change anything about the server interceptor, only the client

[cnnradams]

resolved: need to add interceptor before adding your servicer to server instead of after 😬