Send "meta.span_type" field with root spans

[istathar]

The "meta.kind" field should be populated with a value to ensure the Home screen and Traces are managed to their fullest extent. https://docs.honeycomb.io/working-with-your-data/settings/definitions/#metadata-kind

In consultation with Honeycomb, we've confirmed a value of of "root" should be set for root spans:

Kind is a way to annotate spans within a trace so that edges between components can be identified later. Values can be:

• root - the start of a trace, as you say another way to mark the start in addition to a span having no parent
• subroot - the first span from a service within a trace, used in some of Honeycombs built-ins to show you different services within a single environment
• leaf - some of the beelines mark spans that are the last event from a service (maybe the end of a trace, maybe an end to a sub-process, or maybe the last span before an interaction with another service)
• mid - not a root, subroot, or leaf; an interstitial event between a service's beginning of participation in a trace and the end

It's not yet clear just how much value these provide. This patch adds "root" for when we send a span without a parent span.

[istathar]

Having "root" is actually kind of nice. Merging.