types/logger: move RusagePrefixLog to logger package, disable by default

The RusagePrefixLog is rarely useful, hasn't been useful in a long
time, is rarely the measurement we need, and is pretty spammy (and
syscall-heavy). Disable it by default. We can enable it when we're
debugging memory.

types/logger/rusage.go

@@ -0,0 +1,24 @@
+// 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 logger
+
+import (
+	"fmt"
+	"runtime"
+)
+
+// RusagePrefixLog returns a Logf func wrapping the provided logf func that adds
+// a prefixed log message to each line with the current binary memory usage
+// and max RSS.
+func RusagePrefixLog(logf Logf) Logf {
+	return func(f string, argv ...interface{}) {
+		var m runtime.MemStats
+		runtime.ReadMemStats(&m)
+		goMem := float64(m.HeapInuse+m.StackInuse) / (1 << 20)
+		maxRSS := rusageMaxRSS()
+		pf := fmt.Sprintf("%.1fM/%.1fM %s", goMem, maxRSS, f)
+		logf(pf, argv...)
+	}
+}