Gun messages are not handled by GRPC library

[davydog187]

Describe the bug
When connected to a GRPC service as a client, if that service goes down, or the connection is otherwise lost, :gun_down and :gun_up messages will be received from the calling process.

This behavior is a bit unexpected as Gun is an implementation detail of the library, and no where is it documented that these messages should be handled by the consumer of this library

To Reproduce
Steps to reproduce the behavior:

1. Start a GRPC service
2. Connect to it with this library
3. Kill the GRPC service
4. Observe that :gun_down message is received by the client

Expected behavior
Adapter lifecycle messages are abstracted by the library

[mickel8]

Sounds like a duplicate of #337