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>

tsnet/depaware.txt

@@ -170,7 +170,7 @@ tailscale.com/tsnet dependencies: (generated by github.com/tailscale/depaware)
         tailscale.com/net/bakedroots                                 from tailscale.com/ipn/ipnlocal+
      💣 tailscale.com/net/batching                                   from tailscale.com/wgengine/magicsock
         tailscale.com/net/captivedetection                           from tailscale.com/ipn/ipnlocal+
-        tailscale.com/net/connstats                                  from tailscale.com/net/tstun+
+        tailscale.com/net/connstats                                  from tailscale.com/wgengine/netlog
         tailscale.com/net/dns                                        from tailscale.com/ipn/ipnlocal+
         tailscale.com/net/dns/publicdns                              from tailscale.com/net/dns+
         tailscale.com/net/dns/resolvconffile                         from tailscale.com/net/dns+
@@ -234,6 +234,7 @@ tailscale.com/tsnet dependencies: (generated by github.com/tailscale/depaware)
         tailscale.com/types/logger                                   from tailscale.com/appc+
         tailscale.com/types/logid                                    from tailscale.com/ipn/ipnlocal+
         tailscale.com/types/mapx                                     from tailscale.com/ipn/ipnext
+        tailscale.com/types/netlogfunc                               from tailscale.com/net/tstun+
         tailscale.com/types/netlogtype                               from tailscale.com/net/connstats+
         tailscale.com/types/netmap                                   from tailscale.com/control/controlclient+
         tailscale.com/types/nettype                                  from tailscale.com/ipn/localapi+