Timeouts could already be identified as NaN values on
stunstamp_derp_stun_rtt_ns, but we can't use NaN effectively with
promql to visualize them. So, this commit adds a timeouts metric that
we can use with rate/delta/etc promql functions.
Updates tailscale/corp#20689
Signed-off-by: Jordan Whited <jordan@tailscale.com>