Before, spring-cloud-sleuth-zipkin had to import `SamplerAutoConfiguration`
directly to unwind a sampler ordering problem caused by `TraceAutoConfiguration`
defining the default `Sampler` bean.
This fixes that by moving the default `Sampler` to where it belongs
(`SamplerAutoConfiguration`) and having `TraceAutoConfiguration` import
the sampling configuration directly as opposed to relying on auto-configuration
ordering. Finally it removes the mistake of setting `SamplerAutoConfiguration`
as auto-configuration in the first place.
The name `SamplerAutoConfiguration` was left alone because changing it would
interfere with 3rd party code that formerly imported it to correct this issue
in their non-zipkin setups.
Fixes#1618
16fb8e3ac6