promhttp: implement WithXFromContext in terms of WithXFromRequest

[tie]

This change adds WithLabelFromRequest and WithExemplarFromRequest options and updates FromContext counterparts to be a convenience wrappers for these options.

For example, without this change, setting a label based on http.Request.Pattern requires some juggling with context:

var ctxHTTPRequestKey = httpRequestContext{}

type httpRequestContext struct {
	*http.Request
}

func httpPatternFromContext(ctx context.Context) string {
	r := ctx.Value(ctxHTTPRequestKey).(*httpRequestContext)
	return r.Pattern
}

func instrumentHTTPHandler(h http.Handler) http.Handler {
	h = promhttp.InstrumentHandlerCounter(httpRequestsTotal, h,
		promhttp.WithLabelFromCtx("handler", httpPatternFromContext),
	)
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		var c httpRequestContext
		ctx := context.WithValue(r.Context(), ctxHTTPRequestKey, &c)
		r = r.WithContext(ctx)
		c = httpRequestContext{r}
		h.ServeHTTP(w, r)
	})
}

promhttp.WithLabelFromRequest allows to access http.Request directly:

func instrumentHTTPHandler(h http.Handler) http.Handler {
	return promhttp.InstrumentHandlerCounter(httpRequestsTotal, h,
		promhttp.WithLabelFromRequest("handler", func(r *http.Request) string {
			return r.Pattern
		}),
	)
}