metrics: move currentFDs code to the metrics package

Updates #2784

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

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()
+	}
+}