Allow span 'kind' to be set (e.g. CLIENT|SERVER|CONSUMER|PRODUCER)

[steveoliverguild]

The goal here is to be able to identify spans as either 'CLIENT', 'SERVER', 'CONSUMER', or 'PRODUCER' - such that their relationships on a service graph can be mapped, like this:

Since the package doesn't have a Guzzle outbound HTTP request, you only see the 'SERVER' implementation of this new "kind" that can be set on the raw span.