Add ID metadata to gRPC calls, and use it in logs

[carlosms]

Currently the logs for actions started by a gRPC call initialized by an analyzer do not contain any information to link it to a specific event or PR. Maybe we can include some metadata to the NotifyReviewEvent call and have the analyzers send it back when they call GetChanges.

Context in #245 and #181:

The service/git methods now get the log from the context, but the problem here is that the gRPC requests come from the analyzer, and we don't have the event-id or any other field.
I didn't look much into it, so I don't know if there is a way to pass some ID fields to the analyzer and get them back when they call GetFiles/GetChanges without modifying the proto messages.

Grpc allows to send metadata in header or trailer. There are some helpers in go-grpc-middleware for that.
We can include it in our sdk later to pass event-id to an analyzer and send it back.

[bzz]

As discussed today, it may be a good idea to put in backlog so @dpordomingo could take over this one