trace/Span: SpanKind is missing

[odeke-em]

I am working on another iteration on the website that shows users each concept and usage in the various languages and so far for Spans I can't seem to find any reference of SpanKind in this project.

SpanKind is defined at https://github.com/census-instrumentation/opencensus-proto/blob/99162e4df59df7e6f54a8a33b80f0020627d8405/src/opencensus/proto/trace/v1/trace.proto#L88-L106

Thank you.