Added support for customization of HttpTracing

with this change you can define beans that will get added / set to the `HttpTracing` bean

    fixes gh-886

docs/src/main/asciidoc/spring-cloud-sleuth.adoc

@@ -762,11 +762,31 @@ Running the preceding method with a value of `15` leads to setting a tag with a
 
 //=== Spring Integration
 
-// TODO: Update this
+=== HTTP
 
-// === HTTP
+If a customization of client / server parsing of the HTTP related spans is required,
+just register a bean of type `brave.http.HttpClientParser` or
+`brave.http.HttpServerParser`. If client /server sampling is required, just
+register a bean of type `brave.http.HttpSampler` and name the bean
+ `sleuthClientSampler` for client sampler and `sleuthServerSampler` for server sampler.
+ For your convenience the `@ClientSampler` and `@ServerSampler`
+ annotations can be used to inject the proper beans or to
+ reference the bean names via their static String `NAME` fields.
 
-// TODO: Update this
+Check out Brave's code to see an example of how to make a path-based sampler
+https://github.com/openzipkin/brave/tree/master/instrumentation/http#sampling-policy
+
+If you want to completely rewrite the `HttpTracing` bean you can use the `SkipPatternProvider`
+interface to retrieve the URL `Pattern` for spans that should be not sampled. Below you can see
+an example of usage of `SkipPatternProvider` inside a server side, `HttpSampler`.
+
+[source,java]
+----
+@Configuration
+class Config {
+include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/web/TraceFilterWebIntegrationTests.java[tags=custom_server_sampler,indent=2]
+}
+----
 
 === `TraceFilter`