Set all counter values to zero on start

[hiaselhans]

Thanks for this exporter!

this PR solves #47

it goes through all counters and increases the not-matched counters by zero so they are populated with the corresponding labels and an unchanged value (0 if unset).

to make celery-exporter work i had to remove the git-dependency to celery.
That solves #71
Is the "allow-heartbeats-in-tests" used somewhere?

and then there is this: pallets/markupsafe#282
so i pinned the version in pyproject.toml

[danihodovic]

Is the "allow-heartbeats-in-tests" used somewhere?

It was a branch I made to Celery so that we can test heartbeats. It was merged here celery/celery#6632

Thank you for the contribution Simon

[hiaselhans]

wow, thanks for the fast response!