new plugin: tracing

HoneyryderChuckrequested to merge
tracing-plugin into master

this plugin provides the building blocks that all tracing integrations should build on top of. all they have to do is pass a tracer object to the options, which follows the tracing API:

  • #enabled?(request)
  • #start(request)
  • #finish(request, response)
  • #reset(request)

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