[client][metrics] Add http tracing to generic http client

[at15]

• use net/http/httptrace https://golang.org/pkg/net/http/httptrace/
• return protol, https, http v1 or v2

Ref

• https://blog.golang.org/http-tracing
• https://github.com/davecheney/httpstat/blob/master/main.go#L207
• https://github.com/rakyll/hey/blob/master/requester/requester.go#L141