Streaming interceptor that runs on end of stream

[tsloughter]

Related to #2.

Right now the interceptor is only run when the user calls a grpcbox_client:recv_ function. This breaks the functionality of interceptors like opencensus that rely on the time difference to report span duration and metrics.

Maybe keeping those metrics and exposing them to interceptors would work or have the intercepts run in the stream process, which may come out of #2.