Fix: avgHistogramPercentile / sumHistogramPercentile meter functions reported the smallest finite bucket boundary

[wankai123]

• Add a unit test to verify that the fix works.

• Explain briefly why the bug exists and how to fix it.

• If this pull request closes/resolves/fixes an existing issue, replace the issue number. Closes #.

• Update the CHANGES log.

[Copilot]

Pull request overview

This PR fixes an edge case in the meter histogram percentile implementations (avgHistogramPercentile and sumHistogramPercentile) where an empty window (all buckets = 0) incorrectly produced the smallest finite bucket boundary for every percentile rank. It does so by short-circuiting calculate() to emit 0 percentile values when the bucket total is 0, and it adds unit tests to prevent regressions.

Changes:

• Short-circuit percentile calculation to return 0 for all ranks when the windowed histogram total is 0 (no observed samples).
• Add unit tests covering “no data” scenarios with and without labels for both avg/sum histogram percentile functions.
• Document the bug and fix in the CHANGES log.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/analysis/meter/function/sum/SumHistogramPercentileFunction.java	Short-circuits percentile calculation when total bucket count is 0 to avoid selecting the first bucket boundary.
oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/analysis/meter/function/avg/AvgHistogramPercentileFunction.java	Applies the same “total==0 => all ranks report 0” behavior to avg-based histogram percentiles.
oap-server/server-core/src/test/java/org/apache/skywalking/oap/server/core/analysis/meter/function/sum/SumHistogramPercentileFunctionTest.java	Adds regression tests for no-data histogram percentile behavior (with/without labels).
oap-server/server-core/src/test/java/org/apache/skywalking/oap/server/core/analysis/meter/function/avg/AvgHistogramPercentileFunctionTest.java	Adds regression tests for no-data histogram percentile behavior (with/without labels).
docs/en/changes/changes.md	Records the bug and the fix for release notes.

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