Add tracing option (#487)

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.

middleware/trace/README.md

@@ -0,0 +1,47 @@
+# trace
+
+This module enables OpenTracing-based tracing of DNS requests as they go through the
+middleware chain.
+
+## Syntax
+
+~~~
+trace [ENDPOINT-TYPE] [ENDPOINT]
+~~~
+
+For each server you which to trace.
+
+It optionally takes the ENDPOINT-TYPE and ENDPOINT. The ENDPOINT-TYPE defaults to
+`zipkin` and the ENDPOINT to `localhost:9411`. A single argument will be interpreted as
+a Zipkin ENDPOINT.
+
+The only ENDPOINT-TYPE supported so far is `zipkin`. You can run Zipkin on a Docker host
+like this:
+
+```
+docker run -d -p 9411:9411 openzipkin/zipkin
+```
+
+For Zipkin, if ENDPOINT does not begin with `http`, then it will be transformed to
+`http://ENDPOINT/api/v1/spans`.
+
+## Examples
+
+Use an alternative Zipkin address:
+
+~~~
+trace tracinghost:9253
+~~~
+
+or
+
+~~~
+trace zipkin tracinghost:9253
+~~~
+
+If for some reason you are using an API reverse proxy or something and need to remap
+the standard Zipkin URL you can do something like:
+
+~~~
+trace http://tracinghost:9411/zipkin/api/v1/spans
+~~~