[enhancement] export reqeust size metrics

[qdongxu]

There're 2 items related with the http request size.

1. caddyhttp.computeApproximateRequestSize : cacluates a normal http request from the Content-Lenght header, does not applicable to Transfer-Encoding: chunked bodies or a websocket connection.
2. Server.logRequest handle the request size in a special way by just logged it as the bytes_read field in the log.

Propose to align the behavior of request size with response size.

1. add a method in the ResponseRecorder interface.

type ResponseRecorder interface {
	http.ResponseWriter
	Status() int
	Buffer() *bytes.Buffer
	Buffered() bool
	Size() int
	ReqSize() int  // propose to export
	WriteResponse() error
}

2. export http.request.size as a replacer as well as http.response.size, the latter was exported after the the upstream response received or aborted.

3. update the synmantic of the prometheus metrics request_size_bytes, by ResponseRecorder.ReqSize() to including all bytes of an http request, e.g. Transfer-Encoding: chunked bodies or a websocket connection.

4. keep the log field bytes_read unchanged to keep backward compatibility.

If this proposal is accepted I'm happy to submit a PR.