main.go

func injectHeadersIntoMetadata(ctx context.Context, req *http.Request) metadata.MD {
	headers := []string{
		"x-request-id",
		"x-b3-traceid",
		"x-b3-spanid",
		"x-b3-parentspanid",
		"x-b3-sampled",
		"x-b3-flags",
		"x-ot-span-context"}

	var pairs []string

	for _, h := range headers {
		if v := req.Header.Get(h); len(v) > 0 {
			pairs = append(pairs, h, v)
		}
	}
	return metadata.Pairs(pairs...)
}