How to receive traces by my own?

[caibirdme]

I want to impl a server to receive otlp trace data just like what the collecotor does, and then do some other things. But I fail to receive the traces(unmarshal error). I spent a lot of time but didn't find why, here's my code:

package main

import (
	"go.opentelemetry.io/collector/pdata/ptrace/ptraceotlp"
	"io"
	"log"
	"net/http"
)

func main() {
	http.HandleFunc("/v1/traces", func(w http.ResponseWriter, r *http.Request) {
		body, err := io.ReadAll(r.Body)
		if err != nil {
			log.Printf("error reading body: %v", err)
			w.WriteHeader(http.StatusBadRequest)
			return
		}
		defer r.Body.Close()
		req := ptraceotlp.NewExportRequest()
		err = req.UnmarshalProto(body)
		if err != nil {
			log.Printf("error proto unmarshal body: %v", err)
			w.WriteHeader(http.StatusBadRequest)
			return
		}
		log.Printf("count: %d\n", req.Traces().SpanCount())
		w.WriteHeader(http.StatusOK)
	})
	http.ListenAndServe(":8788", nil)
}