prometheus client Why can’t you use multiple processes

I try to perform multi-process analysis in the code to analyze the data of a certain file, but the code cannot perform the join normally, and there is no method to perform multiprocessing.Process. May I ask what I did wrong. Thank you very much for reading my question. Below is my test code

#coding:utf-8
from prometheus_client import generate_latest, CollectorRegistry
from prometheus_client.core import CollectorRegistry
from flask import Response,Flask
import multiprocessing
class CustomCollector(object):
    def get_data(self,data):
        print(data)

    def collect(self):
        plist = []
        for x in range(30):
            p = multiprocessing.Process(target=self.get_data,args=("test",))
            plist.append(p)
            p.start()

        for p in plist:
            p.join()
        print('Process close')

def rundata():
    REGISTRY = CollectorRegistry(auto_describe=False)
    REGISTRY.register(CustomCollector())
    coll = CustomCollector()
    coll.collect()
    return Response(generate_latest(REGISTRY), mimetype="text/plain")

rundata()

Read more here: https://stackoverflow.com/questions/66325465/prometheus-client-why-cant-you-use-multiple-processes