net/connstats: make it modular (omittable)

Saves only 12 KB, but notably removes some deps on packages that future
changes can then eliminate entirely.

Updates #12614

Change-Id: Ibf830d3ee08f621d0a2011b1d4cd175427ef50df
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/connstats/stats.go

@@ -1,6 +1,8 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
+//go:build !ts_omit_connstats
+
 // Package connstats maintains statistics about connections
 // flowing through a TUN device (which operate at the IP layer).
 package connstats

net/connstats/stats_omit.go

@@ -0,0 +1,24 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build ts_omit_connstats
+
+package connstats
+
+import (
+	"context"
+	"net/netip"
+	"time"
+)
+
+type Statistics struct{}
+
+func NewStatistics(maxPeriod time.Duration, maxConns int, dump func(start, end time.Time, virtual, physical any)) *Statistics {
+	return &Statistics{}
+}
+
+func (s *Statistics) UpdateTxVirtual(b []byte)                                                {}
+func (s *Statistics) UpdateRxVirtual(b []byte)                                                {}
+func (s *Statistics) UpdateTxPhysical(src netip.Addr, dst netip.AddrPort, packets, bytes int) {}
+func (s *Statistics) UpdateRxPhysical(src netip.Addr, dst netip.AddrPort, packets, bytes int) {}
+func (s *Statistics) Shutdown(context.Context) error                                          { return nil }