Resolve discrepancy between JDK attributes coming from different JDK versions

There is a discrepancy between JDK attributes values coming from different JDK versions and JMC is unable to handle it while exposing core API.

For ex:

1. switchRate attribute in JDK 11 onwards have the content type as frequency but pre JDK 11 the content type was number.
2. Pre JDK 9 the attribute name was javalangthread whereas post JDK 9 its thread.

Everything works fine on JMC UI but the issue exists while using core APIs.