Resolving #21, Enable adding custom trace spans from other plugins

[fsanchezgl]

This is a way to enable adding custom span tags to the DD traces as reported in issue #21

The code brings the table exposes through kong.ctx.shared.traces and add the values to the request_span

[dmehala]

Hey @fsanchezgl , Thanks for your pull request. We've found another solution (#28) for your initial request, so we're closing this PR.