MINOR: stats: compute the ratio of compressed response based on 2xx responses

MINOR: stats: compute the ratio of compressed response based on 2xx responses

Since only responses with status 200 can be compressed, let's only count the
ratio of compressed responses on the basis of the 2xx responses and not all
of them. Note that responses 206 are still included in this count but it gives
a better figure, especially for places where authentication is used and 401 is
common.