http2: fire httptrace.ClientTrace.WroteHeaderField if set

ClientTrace.WroteHeaderField was added in Go 1.11.

Updates golang/go#19761 (fixes after vendor into std)

Change-Id: I9a7af31b8601b9cd6efdee63d31a6c05102473d2
Reviewed-on: https://go-review.googlesource.com/122816
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

http2/go111.go

+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build go1.11
+
+package http2
+
+func traceHasWroteHeaderField(trace *clientTrace) bool {
+	return trace != nil && trace.WroteHeaderField != nil
+}
+
+func traceWroteHeaderField(trace *clientTrace, k, v string) {
+	if trace != nil && trace.WroteHeaderField != nil {
+		trace.WroteHeaderField(k, []string{v})
+	}
+}

http2/not_go111.go

+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !go1.11
+
+package http2
+
+func traceHasWroteHeaderField(trace *clientTrace) bool { return false }
+
+func traceWroteHeaderField(trace *clientTrace, k, v string) {}

http2/transport.go

@@ -1291,9 +1291,16 @@ func (cc *ClientConn) encodeHeaders(req *http.Request, addGzipHeader bool, trail
 		return nil, errRequestHeaderListSize
 	}
 
+	trace := requestTrace(req)
+	traceHeaders := traceHasWroteHeaderField(trace)
+
 	// Header list size is ok. Write the headers.
 	enumerateHeaders(func(name, value string) {
-		cc.writeHeader(strings.ToLower(name), value)
+		name = strings.ToLower(name)
+		cc.writeHeader(name, value)
+		if traceHeaders {
+			traceWroteHeaderField(trace, name, value)
+		}
 	})
 
 	return cc.hbuf.Bytes(), nil