improve prometheus metrics

[chipitsine]

when haproxy is built with libslz (not zlib), the following metrics are irrelevant

# HELP haproxy_process_current_zlib_memory Amount of memory currently used by HTTP compression on the current worker process (in bytes)
# TYPE haproxy_process_current_zlib_memory gauge
haproxy_process_current_zlib_memory NaN
# HELP haproxy_process_max_zlib_memory Limit on the amount of memory used by HTTP compression above which it is automatically disabled (in bytes, see global.maxzlibmem)
# TYPE haproxy_process_max_zlib_memory gauge
haproxy_process_max_zlib_memory NaN

we need not to add that metric if haproxy is built with slz