derp: plumb '/derp' request context through (#5083)

This change is required to implement tracing for derp.

Signed-off-by: Charlotte Brandhorst-Satzkorn <charlotte@tailscale.com>

derp/derphttp/derphttp_server.go

@@ -56,6 +56,6 @@ func Handler(s *derp.Server) http.Handler {
 				pubKey.UntypedHexString())
 		}
 
-		s.Accept(netConn, conn, netConn.RemoteAddr().String())
+		s.Accept(r.Context(), netConn, conn, netConn.RemoteAddr().String())
 	})
 }