[feature]combine expvar into grpc-gateway

[jinleileiking]

Is there any way to combine expvar to grpc-gateway for debug?

init func of expvar:

- func init() {
|     http.HandleFunc("/debug/vars", expvarHandler)
|     Publish("cmdline", Func(cmdline))
|     Publish("memstats", Func(memstats))
| }

[jinleileiking]

use expvars.Handler() to realized...