Various trace improvements (#527)

middleware/trace/README.md

@@ -5,26 +5,42 @@ middleware chain.
 
 ## Syntax
 
+The simplest form is just:
+
 ~~~
 trace [ENDPOINT-TYPE] [ENDPOINT]
 ~~~
 
-For each server you which to trace.
+* **ENDPOINT-TYPE** is the type of tracing destination. Currently only `zipkin` is supported
+  and that is what it defaults to.
+* **ENDPOINT** is the tracing destination, and defaults to `localhost:9411`. For Zipkin, if
+  ENDPOINT does not begin with `http`, then it will be transformed to `http://ENDPOINT/api/v1/spans`.
 
-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.
+With this form, all queries will be traced.
 
-The only ENDPOINT-TYPE supported so far is `zipkin`. You can run Zipkin on a Docker host
-like this:
+Additional features can be enabled with this syntax:
+
+~~~
+trace [ENDPOINT-TYPE] [ENDPOINT] {
+	every AMOUNT
+	service NAME
+	client_server
+}
+~~~
+
+* `every` **AMOUNT** will only trace one query of each AMOUNT queries. For example, to trace 1 in every
+  100 queries, use AMOUNT of 100. The default is 1.
+* `service` **NAME** allows you to specify the service name reported to the tracing server.
+  Default is `coredns`.
+* `client_server` will enable the `ClientServerSameSpan` OpenTracing feature.
+
+## 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:
@@ -45,3 +61,13 @@ the standard Zipkin URL you can do something like:
 ~~~
 trace http://tracinghost:9411/zipkin/api/v1/spans
 ~~~
+
+Trace one query every 10000 queries, rename the service, and enable same span:
+
+~~~
+trace tracinghost:9411 {
+	every 10000
+	service dnsproxy
+	client_server
+}
+~~~