metrics: move currentFDs code to the metrics package

Updates #2784

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

metrics/fds_linux.go

@@ -0,0 +1,13 @@
+// Copyright (c) 2021 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 metrics
+
+import "os"
+
+func currentFDs() int {
+	// TODO(bradfitz): do this without so many allocations on Linux.
+	ents, _ := os.ReadDir("/proc/self/fd")
+	return len(ents)
+}

metrics/fds_notlinux.go

@@ -0,0 +1,10 @@
+// Copyright (c) 2021 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.
+
+//go:build !linux
+// +build !linux
+
+package metrics
+
+func currentFDs() int { return 0 }

metrics/metrics.go

@@ -47,3 +47,10 @@ func (m *LabelMap) GetFloat(key string) *expvar.Float {
 	m.AddFloat(key, 0.0)
 	return m.Map.Get(key).(*expvar.Float)
 }
+
+// CurrentFDs reports how many file descriptors are currently open.
+//
+// It only works on Linux. It returns zero otherwise.
+func CurrentFDs() int {
+	return currentFDs()
+}

metrics/metrics_test.go

@@ -0,0 +1,28 @@
+// Copyright (c) 2021 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 metrics
+
+import (
+	"runtime"
+	"testing"
+)
+
+func TestCurrentFileDescriptors(t *testing.T) {
+	if runtime.GOOS != "linux" {
+		t.Skipf("skipping on %v", runtime.GOOS)
+	}
+	if n := CurrentFDs(); n < 3 {
+		t.Errorf("got %v; want >= 3", n)
+	} else {
+		t.Logf("got %v", n)
+	}
+}
+
+func BenchmarkCurrentFileDescriptors(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		_ = CurrentFDs()
+	}
+}