Support service types

[tomverran]

Datadog supports sending one of a few types to tell it what kind of app a particular service is, for example is it a database or is it an API. This doesn't affect very much but changes a few labels in the UI.

I incorrectly thought the span type was a tag but in fact it is a field you need to set in CompletedSpan as per the docs: https://docs.datadoghq.com/api/?lang=python#tracing

The tricky part of this is deciding how to get this type information through Natchez into Datadog given that Natchez only allows you to identify spans by a single string (currently we encode a name/service/resource into this string by using colons to separate them, do we need to add an optional fourth type?)