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

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

log/logheap/logheap_test.go

@@ -0,0 +1,40 @@
+// 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
+
+import (
+	"bytes"
+	"compress/gzip"
+	"encoding/json"
+	"io/ioutil"
+	"testing"
+)
+
+func TestLogHeap(t *testing.T) {
+	var buf bytes.Buffer
+	if err := logHeap(&buf); err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("Got line: %s", buf.Bytes())
+
+	var ll logLine
+	if err := json.Unmarshal(buf.Bytes(), &ll); err != nil {
+		t.Fatal(err)
+	}
+
+	zr, err := gzip.NewReader(bytes.NewReader(ll.Pprof.Heap))
+	if err != nil {
+		t.Fatal(err)
+	}
+	rawProto, err := ioutil.ReadAll(zr)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Just sanity check it. Too lazy to properly decode the protobuf. But see that
+	// it contains an expected sample name.
+	if !bytes.Contains(rawProto, []byte("alloc_objects")) {
+		t.Errorf("raw proto didn't contain `alloc_objects`: %q", rawProto)
+	}
+}