Fix metrics-core-jars version to 2.1.2

Fix metrics-core-jars to version 2.1.2 since the gem has not been working since the upgrade to 3.0.2.
In general it's better to let the maintainer decide when to bump the version. It could be that while this PR was in the queue the version was bumped anyway, or even bumped to something higher, or this PR could be included in something that the maintainer considered requires a minor version bump instead of just a patch.