Actually the server plugin does not store the highest connection count, but MySQL does.
The check you are referring to (mysql.connections) picks the following two values
Max_used_connections 1
max_connections 151
from the ouput of mysql ... -sN -e "show global status ; show global variables ;" and relates them to each other. In my case 1 of 151 is 0.66% and that’s what it shows. The WARN/CRIT levels default to 3 and 5.
Are you asking how to reset the counter in MySQL or which service are you talking about?
You may try mysqladmin flush-status to reset the counters, see the docs.