Expose context methods for current trace and span

[polotek]

Expose the current trace and current span in the top level scope. These properties are useful for doing custom instrumentation.

[ajvondrak]

Hey, just adding my thoughts since these APIs are relevant to my interests. 😄 (Cf. all the talking to myself in #45.)

[grncdr]

Just want to add that since upgrading from 0.8, I couldn't find any "official" way of creating a tracing header for propagation, and ended up with this code which is using send to hack around private methods:

trace_header = Honeycomb.client.send(:context).current_span.try(&:to_trace_header)

So any PR that lets me avoid that would be most welcome 👍

[martin308]

This looks great. Would love to have a little bit more discussion on exposing the non current_span and current_trace accessors. We could split that into another PR so we can discuss those features and get current_span and current_trace released?

[martin308]

@grncdr the way I was expecting people to use the trace header stuff was that you would have a span wrapping the http request so you would end up with something like the faraday integration https://github.com/honeycombio/beeline-ruby/blob/master/lib/honeycomb/integrations/faraday.rb#L25.

[ajvondrak]

Would love to have a little bit more discussion on exposing the non current_span and current_trace accessors. We could split that into another PR so we can discuss those features and get current_span and current_trace released?

Big 👍 on that.

[polotek]

This PR is super simple now. Just exposing the current_trace and current_span. I think there's a lot more to discuss. But if we can land this, it'll enable a lot of good things that aren't possible today.