Add spn_ctx to header

[rdilruba]

Adding spn_ctx to event metadata is discussed in this issue. The span_context in the event is to correlate the event production with what the event consumers are doing. However, it is also possible to add spn_ctx to headers. The HTTP header is for when you need to correlate the process submitting the event with the processing on Nakadi side. It is useful to integrate Nakadi as a part of your trace.