Fix adaptive metrics decay when provider metrics are not updated

[SURYAS1306]

What is the purpose of the change?

This PR fixes an issue in AdaptiveLoadBalance / AdaptiveMetrics where latency decay behaves incorrectly when provider metrics are not updated for a period of time.

Currently, when no new provider metrics arrive, getLoad() may repeatedly apply the penalty branch or aggressively right-shift lastLatency, which can result in stale or extreme values dominating EWMA. This makes adaptive load balancing unstable, especially in low-QPS or intermittent-update scenarios.

This PR ensures that latency decays safely and progressively instead of collapsing or being stuck at penalty values.

Fixes #15810

What is changed?

1. Improved decay logic in AdaptiveMetrics#getLoad()

• Prevents lastLatency from collapsing to zero.
• Ensures decay happens smoothly when provider metrics are not refreshed.
• Avoids repeatedly applying the penalty path when timestamps are equal.

2. Added unit test

Added testAdaptiveMetricsDecayWithoutProviderUpdate

Verifies that when provider metrics are not updated:

• latency decays over time
• penalty value is not stuck
• EWMA continues to evolve

Why is this needed?

Adaptive load balancing relies on EWMA latency to reflect recent performance trends.

Without this fix:

• old latency values can dominate indefinitely
• penalty values may be repeatedly re-applied
• low-traffic services become unfairly weighted

This change makes adaptive load balancing more stable, realistic, and robust under real-world traffic patterns.

Verifying this change

• Added new unit test covering the decay scenario
• All tests pass locally:

mvn -pl dubbo-cluster -am test

Checklist

• Related GitHub issue referenced (Fixes [BUG] Questions about Dubbo Adaptive Load Balance #15810)
• Logic change explained clearly
• Unit test added
• All tests passing locally

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 60.73%. Comparing base (be40746) to head (8946d40).

Files with missing lines	Patch %	Lines
...ain/java/org/apache/dubbo/rpc/AdaptiveMetrics.java	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                3.3   #16048      +/-   ##
============================================
- Coverage     60.74%   60.73%   -0.01%     
+ Complexity    11754    11745       -9     
============================================
  Files          1949     1949              
  Lines         88898    88898              
  Branches      13407    13407              
============================================
- Hits          53998    53993       -5     
- Misses        29337    29342       +5     
  Partials       5563     5563              

Flag	Coverage Δ
integration-tests-java21	32.23% <0.00%> (+<0.01%)	⬆️
integration-tests-java8	32.36% <0.00%> (-0.03%)	⬇️
samples-tests-java21	32.11% <0.00%> (-0.02%)	⬇️
samples-tests-java8	29.84% <0.00%> (+0.14%)	⬆️
unit-tests-java11	59.00% <50.00%> (-0.01%)	⬇️
unit-tests-java17	58.49% <50.00%> (-0.03%)	⬇️
unit-tests-java21	58.51% <50.00%> (+0.01%)	⬆️
unit-tests-java25	58.45% <50.00%> (-0.01%)	⬇️
unit-tests-java8	59.00% <50.00%> (+<0.01%)	⬆️

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[SURYAS1306]

Hi maintainers,

This PR fixes the adaptive metrics decay issue when provider metrics are not updated and adds a unit test covering the scenario.

All checks are green. I’d really appreciate your review when you have time.

Thanks!