Changelog for 2.1.0

The following sections list the changes for 2.1.0.

Summary

Change #258: Switch to official logging library
Since there have been a structured logger part of the Go standard library we
thought it's time to replace the library with that. Be aware that log messages
should change a little bit.
#258
Change #272: Add type to IP pricing and add metrics for primary IPs
Since the client SDK has deprecated the previous handling for the pricing of IP
addresses we had to update the metrics to include the type and location of the
IPs. Besides that we have also added metrics for the pricing of the primary IP
addresses.
#272