feat(metrics): add histogram impl && add table scan metric

[SGZW]

Purpose

• Add histogram metric support to the public Metrics API by introducing HistogramStats and histogram operations (ObserveHistogram, GetHistogramStats, GetAllHistogramStats).
• Implement histogram collection and windowed aggregation (percentiles and stddev estimation), and integrate histogram storage/merge/serialization in MetricsImpl.
• Enhance FileStoreScan scan metrics during plan creation (FileStoreScan::CreatePlan):
• Capture plan-building duration in milliseconds and store it as a counter (lastScanDuration) and a histogram sample (scanDuration) for repeated observations
(src/paimon/core/operation/file_store_scan.cpp:166).
• Introduce the histogram metric key ScanMetrics::SCAN_DURATION (src/paimon/core/operation/metrics/scan_metrics.h:24).

Tests

• UT: histogram and scan-metrics related unit tests are updated/covered.
• IT: N/A

API and Format

• API: Yes. Public metrics API is extended in include/paimon/metrics.h:31.
• Storage format / protocol: No.

Documentation

• Introduces a new metrics capability (histogram). No additional documentation is added in this PR.

[SGZW]

@lucasfang @lxy-9602 PTAL, thanks!

[Copilot]

Pull request overview

Adds histogram support to Paimon’s public Metrics API and wires a new scan-plan duration histogram metric into FileStoreScan so repeated plan creations can be analyzed via percentiles/stddev.

Changes:

• Extend public Metrics API with histogram operations and HistogramStats.
• Implement histogram collection (bucketed) plus windowed aggregation and integrate into MetricsImpl (merge/overwrite/JSON serialization).
• Record scan plan build duration as both a counter (lastScanDuration) and histogram sample (scanDuration), and add unit tests.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
include/paimon/metrics.h	Public API extension: HistogramStats + histogram methods on Metrics.
src/paimon/common/metrics/metrics_impl.h / .cpp	Implements histogram storage, observation, merge/overwrite, and JSON flattening.
src/paimon/common/metrics/histogram.h / .cpp	Adds internal bucketed histogram implementation and stats estimation.
src/paimon/common/metrics/histogram_windowing.h / .cpp	Adds time-windowed histogram wrapper for recent-window aggregation.
src/paimon/core/operation/metrics/scan_metrics.h	Adds ScanMetrics::SCAN_DURATION histogram metric key.
src/paimon/core/operation/file_store_scan.cpp	Records scan plan build duration into counter + histogram.
src/paimon/core/operation/key_value_file_store_scan_test.cpp	Verifies scan duration counter + histogram stats behavior.
src/paimon/core/operation/append_only_file_store_scan_test.cpp	Verifies scan duration counter + histogram stats behavior (append-only).
src/paimon/common/metrics/histogram_test.cpp	Unit tests for histogram correctness + MetricsImpl histogram integration.
src/paimon/common/metrics/histogram_windowing_test.cpp	Unit tests for windowing advancement/reset/merge/clone behavior.
src/paimon/CMakeLists.txt	Adds new histogram sources and tests to the build.
LICENSE	Notes RocksDB inspiration for histogram utilities.

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[SGZW]

@lxy-9602 updated，ptal，thanks

[lucasfang]

+1