THRIFT-5164: Small cleanup on example code

Client: go

Godoc requires at least one other exported type/function to render the
whole example file in the example, so export
simpleProcessorLoggingMiddleware to make the example of
ProcessorMiddleware more helpful.

Currently it's rendered in a not very helpful way:
https://pkg.go.dev/github.com/apache/thrift@v0.13.1-0.20200430141240-5cffef964a08/lib/go/thrift?tab=doc#example-ProcessorMiddleware

Compare to the client middleware example rendering:
https://pkg.go.dev/github.com/apache/thrift@v0.13.1-0.20200430141240-5cffef964a08/lib/go/thrift?tab=doc#example-ClientMiddleware

While I'm here, also update CHANGES.md to mention ClientMiddleware.

[skip ci]
3 files changed