feat: add flatPaths optimization for JSON parsing

[kazeburo]

PR Type

Enhancement, Bug fix, Tests

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Description

• Optimize parsing of small root-level JSON path sets.

• Preserve generic parsing for unsupported or malformed input.

• Propagate percentile append errors.

• Add parity tests and comparative benchmarks.

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Diagram Walkthrough

flowchart LR
  A["Configured JSON paths"]
  B["Flat root-level parser"]
  C["Generic EachKey parser"]
  D["Aggregators"]
  A -- "1–64 root-level paths" --> B
  A -- "other paths" --> C
  B -- "matched values" --> D
  B -- "parse error fallback" --> C
  C -- "remaining values" --> D

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File Walkthrough

	Relevant files
Enhancement	main.go Track eligibility for flat JSON path parsing                          cmd/mackerel-plugin-jsonl/main.go Add flatPaths to parser options. +1/-0      parser.go Parse flat JSON paths with early termination                          cmd/mackerel-plugin-jsonl/parser.go Parse eligible root-level keys with ObjectEach, stopping once all requested paths are found. Fall back to EachKey on parsing errors without delivering already matched values twice. +39/-1    reader.go Select flat paths and propagate append errors                        cmd/mackerel-plugin-jsonl/reader.go Enable flat parsing only for one to 64 single-segment paths. Return errors from percentile append operations. +9/-1
Tests	main_test.go Benchmark flat parsing against generic key parsing              cmd/mackerel-plugin-jsonl/main_test.go Add a benchmark using the generic EachKey parser for comparison. Use require assertions in benchmark setup and validation. +17/-21  parser_test.go Test flat parser parity and path eligibility                          cmd/mackerel-plugin-jsonl/parser_test.go Compare flat parsing with EachKey across escaped, duplicate, missing, compound, and array cases. Verify flat parsing is selected only for one to 64 root-level paths. +104/-0

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[github-actions]

PR Reviewer Guide 🔍

(Review updated until commit f28aee7)

Here are some key observations to aid the review process:

⏱️ Estimated effort to review: 2 🔵🔵⚪⚪⚪

🧪 PR contains tests

🔒 No security concerns identified

⚡ No major issues detected

[github-actions]

Benchmark Comparison

Base: main
PR: feat/flatjson-optimization

BenchmarkMainParse results

goos: linux
goarch: amd64
pkg: github.com/monitoring-forge/mackerel-plugin-jsonl/cmd/mackerel-plugin-jsonl
cpu: AMD EPYC 7763 64-Core Processor                
                             │ base-bench.txt │            pr-bench.txt             │
                             │     sec/op     │   sec/op     vs base                │
MainParse_jsonl-4                42.97m ± 10%   34.14m ± 1%  -20.53% (p=0.000 n=10)
MainParse_parse_and_output-4     44.67m ±  2%   35.93m ± 3%  -19.57% (p=0.000 n=10)
MainParse_jsonl_eachkey-4                       42.47m ± 1%
geomean                          43.81m         37.35m       -20.06%

                             │ base-bench.txt │            pr-bench.txt             │
                             │      B/op      │     B/op      vs base               │
MainParse_jsonl-4                4.642Mi ± 0%   4.642Mi ± 0%       ~ (p=0.315 n=10)
MainParse_parse_and_output-4     5.412Mi ± 0%   5.412Mi ± 0%  +0.00% (p=0.000 n=10)
MainParse_jsonl_eachkey-4                       4.642Mi ± 0%
geomean                          5.012Mi        4.885Mi       +0.00%

                             │ base-bench.txt │            pr-bench.txt            │
                             │   allocs/op    │  allocs/op   vs base               │
MainParse_jsonl-4                 100.4k ± 0%   100.4k ± 0%       ~ (p=0.345 n=10)
MainParse_parse_and_output-4      100.5k ± 0%   100.5k ± 0%  +0.00% (p=0.000 n=10)
MainParse_jsonl_eachkey-4                       100.4k ± 0%
geomean                           100.5k        100.4k       -0.00%

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Persistent review updated to latest commit f28aee7

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