net/http/httptrace: add simple example and fix copyright header

Partially addresses #16360

Change-Id: I67a328302d7d91231f348d934e4232fcb844830a
Reviewed-on: https://go-review.googlesource.com/27398Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/httptrace/example_test.go

+// Copyright 2016 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.
+
+package httptrace_test
+
+import (
+	"fmt"
+	"net/http"
+	"net/http/httptrace"
+)
+
+func ExampleTrace() {
+	req, _ := http.NewRequest("GET", "http://example.com", nil)
+	trace := &httptrace.ClientTrace{
+		GotConn: func(connInfo httptrace.GotConnInfo) {
+			fmt.Printf("Got Conn: %+v\n", connInfo)
+		},
+		DNSDone: func(dnsInfo httptrace.DNSDoneInfo) {
+			fmt.Printf("DNS Info: %+v\n", dnsInfo)
+		},
+	}
+	req = req.WithContext(httptrace.WithClientTrace(req.Context(), trace))
+	http.DefaultClient.Do(req)
+}

src/net/http/httptrace/trace.go

 // Copyright 2016 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.h
+// license that can be found in the LICENSE file.
 
 // Package httptrace provides mechanisms to trace the events within
 // HTTP client requests.

src/net/http/httptrace/trace_test.go

 // Copyright 2016 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.h
+// license that can be found in the LICENSE file.
 
 package httptrace