Fix a wrong condition for the _purge_metering_info function

Fix a wrong condition for the _purge_metering_info function

Fix a situation for the _purge_metering_info function
when the items will never be deleted from the metering_info.
Delete the metering_info dict and use the metering_infos instead.
Fix the problem with changing a dictionary during iteration.
Add the unit tests for the _purge_metering_info and
_add_metering_info functions.

Co-Authored-By: Yaroslav Isakov <yisakov@mirantis.com>
Change-Id: I9031a5f27ae6438ffd5c5a48b0cf5cdc6867eff3
Closes-Bug: #1490581