Fix process metric precision

[Caideyipi]

Summary

• Preserve fractional values in the process CPU load metric
• Preserve fractional values in the process memory ratio metric

This prevents low CPU usage from being reported as zero and avoids rounding process memory utilization to a whole percentage.

Verification

• mvn spotless:apply -pl iotdb-core/datanode
• mvn compile -pl iotdb-core/datanode -DskipTests

[sonarqubecloud]

Quality Gate failed

Failed conditions
B Reliability Rating on New Code (required ≥ A)

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.78%. Comparing base (5b311f6) to head (f3f580f).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
...pache/iotdb/db/service/metrics/ProcessMetrics.java	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #18229   +/-   ##
=========================================
  Coverage     42.78%   42.78%           
  Complexity      343      343           
=========================================
  Files          5356     5356           
  Lines        380277   380294   +17     
  Branches      49332    49332           
=========================================
+ Hits         162691   162715   +24     
+ Misses       217586   217579    -7     

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