improve tracing across micro

[vtolstov]

after complete micro.Tracer interface and allow to use opentracing for it add micro.Tracer to all stuff like

• store
• client
• broker
• server
• auth
• codec?
• config
• events?
• network/transport
• network/tunnel?
• proxy?
• registry
• router
• resolver?
• selector?
• sync?

[vtolstov]

useful for example like https://github.com/go-redis/redis/blob/extra/redisotel/v0.2.0/extra/redisotel/redisotel.go