[Hotfix] Fetch the latest MemoryUsage inside each gauge call

[7hong]

Why are the changes needed?

Brief change log

The registerHeapMetric method only reads the JVM heap usage once at registration time and then reuses that snapshot inside the gauges, which causes the heap metrics to never change after being registered.

How was this patch tested?

• Add some test cases that check the changes thoroughly including negative and positive cases if possible

• Add screenshots for manual tests if appropriate

• Run test locally before making a pull request

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 28.64%. Comparing base (cad9869) to head (3c0451c).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #4062      +/-   ##
============================================
- Coverage     28.79%   28.64%   -0.16%     
+ Complexity     3944     3935       -9     
============================================
  Files           657      653       -4     
  Lines         52431    52276     -155     
  Branches       6664     6622      -42     
============================================
- Hits          15098    14972     -126     
+ Misses        36226    36211      -15     
+ Partials       1107     1093      -14     

Flag	Coverage Δ
core	28.64% <100.00%> (-0.16%)	⬇️

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