Distributed trace for generic (not http) requests

[VitorTrin]

Is your feature request related to a problem? Please describe.

Distributed trace works great for http requests, but there is no (clean) way to make it work when the request isn't related to http.

Describe the solution you'd like

A function that creates a serialized context, most likely DistributedTrace.distributed_trace_headers(:generic), and a function that takes a serialized context and adds to the current transaction, akin to DistributedTrace.track_transaction(context, transport_type: transport_type) but that accepts the serialized context.

Describe alternatives you've considered

The current workaround is to call DistributedTrace.distributed_trace_headers(:http), make them serializeble on our own, add to the request, and the other side takes the serialized headers, builds a conn, adds the headers and calls W3CTraceContext.extract() to get a built context.

[binaryseed]

Duplicate: #265