Add support for OpenTelemetry span kind

[stevapple]

https://github.com/open-telemetry/opentelemetry-specification/blob/52a35899e6ae7e6f85c53aa38bfbfec091b79fac/specification/trace/semantic_conventions/messaging.md?plain=1#L178-L182

This PR adds span.kind=SERVER to "Main Request"s for Jaeger and Zipkin formats, to identify them as the entry point of a server application.

Spans whose kind is undefined are equivalent to INTERNAL, which indicates they are internal processes called by the server.

[Pierre-Lannoy]

Thanks :)