[branch-4.16] Fix ThreadRegistry#register behavior to ensure correct Prom metrics #4300

[zhiheng123]

• Make tests fail

• Fix ThreadRegistry#register to ensure correct Prom metrics

• Change style to match BK standards

• Fix tests

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Co-authored-by: Nicolò Boschi boschi1997@gmail.com

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(cherry picked from commit 1d82b92) Conflicts:
bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/BookieImpl.java
bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/SyncThread.java

Descriptions of the changes in this PR:

Fix #xyz

Main Issue: #xyz

BP: #xyz

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Changes

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