Investigate random grpc spec failures

[estolfo]

I've disabled the grpc spec because it randomly fails. The interceptor feature is experimental in the grpc gem so perhaps the fault is not within our code. We should take a closer look at the test failures and see if we can resolve them.

[Dariusch]

Hey guys, I actually just tested the cerver and client interceptor with bigcommerce/gruf and stumbled opon an error in your Code. I tried to make it work but I wasn't successful.

https://github.com/Dariusch/apm-agent-ruby/commit/8f55136d146ec1d6553986502ecadf8c0b8e07b4

Hope it helps to resolve some Issues