Update CHANGELOG in preparation for 2.2.0 release
Update CHANGELOG in preparation for 2.2.0 release

Includes the changes in `master` that we'll release in `2.2.0`, and
the ones that we can't because of breaking changes, which we'll release
in `3.0.0`

Signed-off-by: Daniel Magliola <349b614faf8a67c5438eb3fc104c4bf440ac9ea6@crystalgears.com>

prometheus/client_ruby