Provide additional key VM stats scoped to a transaction as agent attributes on the transaction

[fallwith]

We have #2352 for a comprehensive look at improving our VM stats offerings. And New Relic could benefit from an even broader look at how it presents VM stats for all languages.

But for this innovation idea, let's keep things smaller and accomplishable with just the Ruby agent code base. Perhaps a predefined list - which can be overridden by a customer specified list - will drive readings that are taken by the transaction creating thread at transaction start time and taken again at transaction completion time. These readings will then be associated with the transaction as agent attributes. The presence of these attributes could drive queries and alerts for the short term and possibly roll back into larger UI backed initiatives later.

[workato-integration]

https://new-relic.atlassian.net/browse/NR-264281