RFR: 7243: Resolve discrepancy between JDK attributes coming from different JDK versions

[Suchita Chaturvedi]

This PR resolves the issue of thread attribute discrepancy between different JDK versions.

The issue had two parts - first one was already resolved as part of another bug, second part will be fixed as part of this PR.

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.

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Commit messages:
 - 7243: Resolve discrepancy between JDK attributes coming from different JDK versions

Changes: https://git.openjdk.org/jmc/pull/411/files
 Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=411&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-7243
  Stats: 15 lines in 1 file changed: 13 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jmc/pull/411.diff
  Fetch: git fetch https://git.openjdk.org/jmc pull/411/head:pull/411

PR: https://git.openjdk.org/jmc/pull/411