max_connections should be included for memcached

[ZhenLiuInstacart]

This should only take a minute!

Memcache config include max_connections and curr_connections. Currently only curr_connections is included. Max_connections is important as well and we do need the ratio of curr/max to make operational decisions. So can we add max_connections to https://github.com/DataDog/integrations-core/blob/master/mcache/datadog_checks/mcache/mcache.py#L27:L37

[FlorentClarret]

Hi @ZhenLiuInstacart, thanks for this issue. I opened a PR to add this metric.