bugfix(opentelemetry-instrumentation-grpc): Add code() and details() to ServicerContext

[CoLa5]

Description

_OpenTelemetryServicerContext aims to implement the abstract interface of grpc.ServicerContext.
grpc.ServicerContext supports since grpcio v1.38.0 (s. here) a code() and details()-method to retrieve the currently set code/details in the servicer method.
In contrast, _OpenTelemetryServicerContext implements code and details as public attributes, which are not callable.
Consequently, calling code() and details() in a servicer method will fail, when using OpenTelemetry, e.g.:

class Greeter(helloworld_pb2_grpc.GreeterServicer):

    def SayHello(self, request, context):
        print("code before: %r" % context.code())  # ERROR
        context.set_code(grpc.StatusCode.OK)
        print("code after: %r" % context.code())  # ERROR

        print("details before: %r" % context.details())  # ERROR
        context.set_details("test")
        print("details after: %r" % context.details())  # ERROR

        return helloworld_pb2.HelloReply(message='Hello, %s!' % request.name)

will raise an error.

Obviously, the abc-module of python allows this bug, since the @abstractmethods code() and details() of grpc.ServicerContext have been overwritten by _OpenTelemetryServicerContext, although the abstract interface has not been implemented correctly.

Fixes (#855)

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Not tested.

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[srikanthccv]

Was there ever a publicly accessible code and details attributes?

[CoLa5]

As said in the PR, grpc.ServicerContext supports since grpcio v1.38.0 (s. here) a code() and details()-method to retrieve the currently-set code/details in the servicer method. Before, both methods did not exist in the public interface.

And starting from the first non-beta version v0.15 of grpc in python, the internal implementation of the grpc.ServicerContext uses an internal/private _RpcState-object to manage the set code/details.

Consequently, code- or details-attributes have never been publicly accessible.