[EXT] `producer` and `consumer` span kinds
[EXT] `producer` and `consumer` span kinds

In https://github.com/opentracing/specification/blob/master/semantic_conventions.md#span-tags-table
it is clearly stated that spans not only consist of RPC client and
server relationships, but also  producer and consumer relationships when
you are in a messaging context.

This PR adds those missing Tag constants as `SPAN_KIND_MESSAGE_BUS_PRODUCER`
and `SPAN_KIND_MESSAGE_BUS_CONSUMER`.

opentracing/opentracing-php