net/tstun, wgengine: add packet-level and drop metrics

Primarily tstun work, but some MagicDNS stuff spread into wgengine.

No wireguard reconfig metrics (yet).

Updates #3307

Change-Id: Ide768848d7b7d0591e558f118b553013d1ec94ad
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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