Memory metric in percentage and bug fixes

Added

• Add memory_in_percentages and swap_in_percentages host metrics that represents metrics in percentages.

Changed

• Ignore /snap/ disk mountpoints.

Fixed

• Fix issue with the open span count in logs being logged as a negative number.
• Fix agent's TCP server getting stuck when two requests are made within the same fraction of a second.

See the Python package 0.3.1 changelog for more information.