When xmetrics.IsEnabled() is false (no -api / no metrics.addr configured),
WrapListener now returns the listener unchanged instead of wrapping every
accepted connection in a metrics.serverConn. This allows bare *net.TCPConn
to reach the forwarding layer, enabling future splice(2) optimization.
Add UnwrapConn() to the admission, proxyproto, conn limiter, metrics,
and observer/stats TCP connection wrappers, complementing the traffic
limiter's limitConn added in the previous commit.
This makes the unwrapConn() helper in handler/sshd able to peel through
any combination of these wrappers uniformly, keeping the pattern
complete and future-proof for handlers that assert on concrete
connection types.
ginuerzh