tailcfg: add MapResponse.Debug mechanism to trigger logging heap pprof

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

log/logheap/logheap.go

@@ -0,0 +1,45 @@
+// Copyright (c) 2020 Tailscale Inc & 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 logheap logs a heap pprof profile.
+package logheap
+
+import (
+	"bytes"
+	"encoding/json"
+	"io"
+	"os"
+	"runtime"
+	"runtime/pprof"
+	"time"
+)
+
+// LogHeap writes a JSON logtail record with the base64 heap pprof to
+// os.Stderr.
+func LogHeap() {
+	logHeap(os.Stderr)
+}
+
+type logTail struct {
+	ClientTime string `json:"client_time"`
+}
+
+type pprofRec struct {
+	Heap []byte `json:"heap,omitempty"`
+}
+
+type logLine struct {
+	LogTail logTail  `json:"logtail"`
+	Pprof   pprofRec `json:"pprof"`
+}
+
+func logHeap(w io.Writer) error {
+	runtime.GC()
+	buf := new(bytes.Buffer)
+	pprof.WriteHeapProfile(buf)
+	return json.NewEncoder(w).Encode(logLine{
+		LogTail: logTail{ClientTime: time.Now().Format(time.RFC3339Nano)},
+		Pprof:   pprofRec{Heap: buf.Bytes()},
+	})
+}