Revert "monitor-config-store: Only save 3 decimal places of <rate>"
The reverted commit started using locale dependent methods to store floating point values in configuration files, which breaks depending on what locale is used.
This also contains revert "tests: Expect to always have three decimal places (rounded)". The latter revert accidentally landed separately meaning the test suite had a broken commit, but this revert reverts both in one commit to avoid that.