Configurable RestTemplate tracing with support for RestTemplateBuilder

[mdvorak]

1. Added WebClientTracingProperties, with enabled and componentName
2. Moved RestTemplate and AsyncRestTemplate AutoConfig into once umbrella class RestTemplateTracingAutoConfiguration, which allows us to have all @Conditional annotations in one place. Actual configurations are in nested static classes, which is pattern used heavily by Spring Boot itself. Allows us to use additional conditions on each configuration class.
3. Added TracingRestTemplateCustomizer, which is used by Spring Boot RestTemplateBuilder (not included in Spring Framework itself). With SB, its valid pattern to have one preconfigured builder bean, and consumers create RestTemplate instances locally, private, even on demand - but still share common configuration. RestTemplateCustomizer is pattern to hook into builder config without directly modifying the bean. Side effect is that also TestRestTemplate now creates traces - therefore disabled client tracing for Servlet tests.

This is a big PR including multiple changes, but I found it would be complicated to split it across several PRs. It should however be non-breaking backwards compatible. Only change in behavior should be for RestTemplateBuilder beans, which might influence some applications. But its likely it will fix them instead of breaking them.

Discussion is welcome :)

[pavolloffay]

@mdvorak thanks for the PR.

The auto-configuration part looks fine.

• can we keep the core module without spring boot classes e.g. RestTemplateBuilder?
• Could you keep the instrumentation classes without changes?

[pavolloffay]

Can be TracingRestTemplateCustomizer moved to the starter module?

[mdvorak]

You mean opentracing-spring-web-autoconfigure? Of course. Moving it to opentracing-spring-cloud would split RestTemplate config into two separate projects, which I don't like. Commit coming up :)

[mdvorak]

Note: While RestTemplateCustomizer should not break anything, its a new feature, and should at least have version bump to 0.3.x.

[mdvorak]

I've tested the change with opentracing-spring-cloud, and indeed there are plenty of failing tests - whenever they expect exact span count in the tracer and TestRestTemplate is used. I can prepare patch for otsc that will work with this version by disabling client tracing for all tests where servlet is tested.

• due to this change, i would really bump version to 0.3.0, what do you think @pavolloffay? I can change the version in PR if you want.

[pavolloffay]

@mdvorak sorry for the delay, I am fine with changes, I will have the last look and merge/comment.

You don't have to create a PR to bump the version, it will be done automatically as part of the release.

[mdvorak]

No problem, thanks for the review, I know its as hard as coding :-)

[pavolloffay]

@mdvorak thanks! I will cut 0.3.0