metrics: move currentFDs code to the metrics package

Updates #2784

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

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 }