The main arguments for these features are
* api-agnostic means to collaborate with a span
- use of annotations allows users to add to a span with no library dependency on a span api.
This allows Sleuth to change its core api less impact to user code.
* reduced surface area for basic span operations.
- without this feature one has to use the span api, which has lifecycle commands that
could be used incorrectly. By only exposing scope, tag and log functionality, users can
collaborate without accidentally breaking span lifecycle.
* collaboration with runtime generated code
- with libraries such as Spring Data / Feign the implementations of interfaces are generated
at runtime thus span wrapping of objects was tedious. Now you can provide annotations
over interfaces and arguments of those interfaces
This PR is an adoption of @Koizumi85 work started here - https://github.com/Koizumi85/spring-cloud-sleuth-annotationfixes#182
without this change there is no support for context propagation
with this change whenever you pass the `baggage-...` for http or `baggage_` for messaging headers then such a value will be propagated through your system
fixes#237
without this change we had to treat every mean of injecting and extracting span information separately
with this change all means of communications are abstracted. That way you configure things once only.
part of #237
* Fixed the HttpServlet extractors
(now the response can contain custom headers)
* Changed header names to be Zipkin compatible
* removed qualifiers and properties
* updated the docs
fixes#19
