RFR: 8344445: MethodCounters don't need a vptr

[Coleen Phillimore]

This is a somewhat trivial change to make MethodCounters inherit from MetaspaceObj so that they don't have any virtual functions (and vptrs).  They're just a bunch of ints.

Tested with tier1-4.

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Commit messages:
 - 8344445: MethodCounters don't need a vptr

Changes: https://git.openjdk.org/jdk/pull/22220/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22220&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344445
  Stats: 14 lines in 3 files changed: 3 ins; 5 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/22220.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22220/head:pull/22220

PR: https://git.openjdk.org/jdk/pull/22220