Saves ~102 KB from the min build.
Updates #12614
Change-Id: Ie1d4f439321267b9f98046593cb289ee3c4d6249
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This required sharing the dropped packet metric between two packages
(tstun and magicsock), so I've moved its definition to util/usermetric.
Updates tailscale/corp#22075
Signed-off-by: Anton Tolchanov <anton@tailscale.com>