Fix compatibility with Sanic 21.12.0+

[jmarcet]

The deprecation is now an error with current Sanic 21.12.0

Would you mind to include the fix?

[anna-hope]

Is there any timeline for merging this?

[anttilip]

In __init__.py lines 134 and 144 app.memcollect_task should be changed to app.ctx.memcollect_task as these are still causing errors.

https://github.com/dkruchinin/sanic-prometheus/blob/24c15860c3e133a416868f15567ccd2577e0d2e1/sanic_prometheus/init.py#L132-L145

[erickg-k]

Are you still working on this? @jmarcet