@Mohammed El Moumni
You can use the rest api to get the value of counter. You could create a script with the needed curl commands the get the value of the counter and then use the ExecuteScript processor to run the script.
You can access the rest api document via your NiFi instance, the url will be similar to this http://nifi-host:port/nifi-docs/rest-api.
For example, curl command to get counters in my flow, with output, is:
curl 'http://nifi-host:port/nifi-api/counters'
{"counters":{"aggregateSnapshot":{"generated":"15:12:55 BST","counters":[{"id":"46f421a7-d348-3e65-8a21-290cf1fc8fa1","context":"ExecuteScript (abde5770-015b-1000-0000-0000722fe055)","name":"my-counter","valueCount":0,"value":"0"},{"id":"bd62b47c-5124-35b1-926b-2149eb519f79","context":"ExecuteScript (abfdad0a-015b-1000-ffff-fffff0fcbeb9)","name":"my-counter","valueCount":56400,"value":"56,400"},{"id":"c6d58dcc-8171-3d63-adb2-8cc8e239cc9a","context":"Groovy script of counter (abfdad0a-015b-1000-ffff-fffff0fcbeb9)","name":"my-counter","valueCount":6930,"value":"6,930"},{"id":"776c5c43-40ee-372e-957c-f40b465d3326","context":"All ExecuteScript's","name":"my-counter"