Update dependencies

vendor/tailscale.com/metrics/fds_linux.go

@@ -0,0 +1,41 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package metrics
+
+import (
+	"io/fs"
+	"sync"
+
+	"go4.org/mem"
+	"tailscale.com/util/dirwalk"
+)
+
+// counter is a reusable counter for counting file descriptors.
+type counter struct {
+	n int
+
+	// cb is the (*counter).count method value. Creating it allocates,
+	// so we have to save it away and use a sync.Pool to keep currentFDs
+	// amortized alloc-free.
+	cb func(name mem.RO, de fs.DirEntry) error
+}
+
+var counterPool = &sync.Pool{New: func() any {
+	c := new(counter)
+	c.cb = c.count
+	return c
+}}
+
+func (c *counter) count(name mem.RO, de fs.DirEntry) error {
+	c.n++
+	return nil
+}
+
+func currentFDs() int {
+	c := counterPool.Get().(*counter)
+	defer counterPool.Put(c)
+	c.n = 0
+	dirwalk.WalkShallow(mem.S("/proc/self/fd"), c.cb)
+	return c.n
+}