logtest.py

from metaflow import FlowSpec, step

class HelloWorldFlow(FlowSpec):

    @step
    def start(self):
        print("This is start step")
        import time
        print("<<BEGIN>> 10")
        for i in range(10):
            print('<<ITER>>')
            time.sleep(1)
            if i == 5:
                print('Show a message while progressbar active')
        print('done!')
        self.next(self.end)

    @step
    def end(self):
        print("This is end step")

if __name__ == '__main__':
    HelloWorldFlow()

mf_tqdm.py

import sys
import re
from tqdm import tqdm

RE = re.compile('(\<\<.+?\>\>)( .+)?')

bar = None
barmax = None
for line in sys.stdin:
    m = RE.search(line)
    if m:
        if m.group(1) == '<<BEGIN>>':
            barmax = int(m.group(2))
            bar = tqdm(leave=False, total=barmax)
        elif bar and m.group(1) == '<<ITER>>':
            bar.update()
            barmax -= 1
            if barmax == 0:
                bar.close()
                bar = None
    elif bar:
        bar.write(line.strip())
    else:
        sys.stdout.write(line)

This is a kind of subtle way to detour for showing progress bar of foreach in metaflow, but it works fine.
Thank you so much!