Putting handlers into log fields makes ZipkinV2Reporter cause Gson errors

[eungju]

1. TracingHandlerInterceptor puts a handler object into log fields. See this.
2. ZipkinV2Reporter converts log fields to JSON via Gson to build JaegerSpan annotations. See this.
3. Gson inspects private fields of the handler object.
4. Some handlers have private fields that can not be handled by Gson.

This makes errors when I use TracingHandlerInterceptor and ZipkinV2Reporter together.

Do we need to put handler objects into log fields? I think class name and method name is enough.