Support multiple tracing systems

[TYsewyn]

Currently one of the requirements I have is to support multiple tracing systems (GCP Stackdriver Trace & Zipkin). Unfortunately it is not possible to use the auto-configuration which comes with the 2 dependencies (spring-cloud-gcp-starter-trace and spring-cloud-starter-sleuth-zipkin).
Supporting both systems also requires to duplicate almost all of the auto-configuration.

It'd be interesting if spring-cloud-sleuth-core comes shipped with an @Primary bean which is a composite of all the configured SpanReporters, or something that is a brave based FinishedSpanHandler (which will be more efficient) like @adriancole said on gitter.

[codefromthecrypt]

yep and to clarify.. right now, no reporter is based on FinishedSpanHandler but anyway the overall design you mention seems to apply regardless

[marcingrzejszczak]

Done via #1181