go-smtp Server-Side Observability

[kingster]

@emersion Bring this discusion back from IRC, what way do you think we can add observalibity of the on the on-wire responses that the go-smtp server is sending?

What do you think about exposing interceptors for the smtp server which would allow any kind of monitoring to be plugged in?

Based on discussion on IRC and #273

[emersion]

Sorry, I don't have time to sit down and design a good API for this at this time.

[kingster]

Let me comeback with a proposal then. Hopefully you can review it :)