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)
+}