* Rename middleware to plugin
first pass; mostly used 'sed', few spots where I manually changed
text.
This still builds a coredns binary.
* fmt error
* Rename AddMiddleware to AddPlugin
* Readd AddMiddleware to remain backwards compat
* checkpoint
* Pass context through ServeDNS, enable gRPC tracing
* Fix types and make tracer available to proxy. go fmt
* Fix imports
* Use the DoNotStartTrace option
* Change to SpanFilter from DoNotStartTrace
* Use new name (IncludeSpan)
* Final names
* Add tests; fix possible client/conn leaks in grpc
* go fmt
Adds a middleware to enable tracing with OpenTracing/OpenZipkin.
Enabling tracing will have a large impact on performance so it is
not advisable in production.