net/connstats: prepare to remove package (#17554)

The connstats package was an unnecessary layer of indirection.
It was seperated out of wgengine/netlog so that net/tstun and
wgengine/magicsock wouldn't need a depenedency on the concrete
implementation of network flow logging.

Instead, we simply register a callback for counting connections.
This PR does the bare minimum work to prepare tstun and magicsock
to only care about that callback.

A future PR will delete connstats and merge it into netlog.

Updates tailscale/corp#33352

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

types/netlogfunc/netlogfunc.go

@@ -0,0 +1,15 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package netlogfunc defines types for network logging.
+package netlogfunc
+
+import (
+	"net/netip"
+
+	"tailscale.com/types/ipproto"
+)
+
+// ConnectionCounter is a function for counting packets and bytes
+// for a particular connection.
+type ConnectionCounter func(proto ipproto.Proto, src, dst netip.AddrPort, packets, bytes int, recv bool)