write-header-callers.patch

diff --git a/vendor/go.elastic.co/apm/module/apmhttp/handler.go b/vendor/go.elastic.co/apm/module/apmhttp/handler.go
index 2614c2c4..169d2fe5 100644
--- a/vendor/go.elastic.co/apm/module/apmhttp/handler.go
+++ b/vendor/go.elastic.co/apm/module/apmhttp/handler.go
@@ -2,6 +2,8 @@ package apmhttp
 
 import (
 	"context"
+	"log"
+	"runtime/debug"
 	"net/http"
 
 	"go.elastic.co/apm"
@@ -164,11 +166,18 @@ type Response struct {
 type responseWriter struct {
 	http.ResponseWriter
 	resp Response
+	writeHeaderCalls [][]byte
 }
 
 // WriteHeader sets w.resp.StatusCode and calls through to the embedded
 // ResponseWriter.
 func (w *responseWriter) WriteHeader(statusCode int) {
+	w.writeHeaderCalls = append(w.writeHeaderCalls, debug.Stack())
+	if len(w.writeHeaderCalls) > 1 {
+		for i, stack := range w.writeHeaderCalls {
+			log.Printf("stack %d:\n%s", i, stack)
+		}
+	}
 	w.ResponseWriter.WriteHeader(statusCode)
 	w.resp.StatusCode = statusCode
 }