[liggitt]

cc @bwplotka

part of #2105

1. Adds a benchmark for apiResult decoding
2. Switches to stdlib for the envelope decoding

Benchmark results of switching the envelope decode to stdlib:

On Go 1.26:

• CPU is about 180% slower than json-iter
• Bytes range from ~flat (on larger complex responses) to ~2x json-iter
• Allocs are consistently better, and way better than json-iter on large complex responses

On Go 1.27:

• CPU is about 35% slower than json-iter
• Bytes are improved 0-38% compared with json-iter
• Allocs are improved 70-100% (!) over json-iter

goos: darwin
goarch: arm64
pkg: github.com/prometheus/client_golang/api/prometheus/v1
cpu: Apple M4 Pro
                                     │    main     │               go1.26.6               │              go1.27.1               │
                                     │   sec/op    │    sec/op     vs base                │    sec/op     vs base               │
APIResponse/AlertsResult-14            364.8n ± 1%   1054.0n ± 1%  +188.93% (p=0.002 n=6)   470.2n ±  1%  +28.88% (p=0.002 n=6)
APIResponse/AlertManagersResult-14     307.2n ± 1%    931.8n ± 1%  +203.34% (p=0.002 n=6)   408.9n ±  2%  +33.11% (p=0.002 n=6)
APIResponse/ConfigResult-14            191.0n ± 0%    577.6n ± 1%  +202.30% (p=0.002 n=6)   303.3n ±  2%  +58.75% (p=0.002 n=6)
APIResponse/FlagsResult-14             187.5n ± 1%    523.5n ± 0%  +179.13% (p=0.002 n=6)   297.6n ±  1%  +58.68% (p=0.002 n=6)
APIResponse/BuildinfoResult-14         311.1n ± 3%    904.2n ± 1%  +190.65% (p=0.002 n=6)   422.6n ±  2%  +35.84% (p=0.002 n=6)
APIResponse/RuntimeinfoResult-14       436.3n ± 2%   1258.0n ± 1%  +188.30% (p=0.002 n=6)   523.2n ±  1%  +19.90% (p=0.002 n=6)
APIResponse/model.LabelValues-14       189.3n ± 1%    546.9n ± 0%  +188.80% (p=0.002 n=6)   300.2n ±  1%  +58.54% (p=0.002 n=6)
APIResponse/SnapshotResult-14          191.6n ± 2%    535.3n ± 1%  +179.38% (p=0.002 n=6)   299.2n ±  1%  +56.18% (p=0.002 n=6)
APIResponse/TargetsResult-14           234.8n ± 1%    644.1n ± 1%  +174.32% (p=0.002 n=6)   331.9n ±  1%  +41.35% (p=0.002 n=6)
APIResponse/[]MetricMetadata-14        310.4n ± 1%    856.9n ± 0%  +176.05% (p=0.002 n=6)   422.1n ±  1%  +35.97% (p=0.002 n=6)
APIResponse/map[string][]Metadata-14   281.9n ± 1%    744.9n ± 1%  +164.29% (p=0.002 n=6)   379.6n ±  1%  +34.66% (p=0.002 n=6)
APIResponse/TSDBResult-14              676.0n ± 1%   1869.5n ± 1%  +176.55% (p=0.002 n=6)   792.2n ±  1%  +17.19% (p=0.002 n=6)
APIResponse/TSDBBlocksResult-14        518.7n ± 1%   1328.0n ± 0%  +156.02% (p=0.002 n=6)   597.2n ±  1%  +15.12% (p=0.002 n=6)
APIResponse/WalReplayStatus-14         247.3n ± 1%    615.8n ± 1%  +148.99% (p=0.002 n=6)   338.6n ± 14%  +36.92% (p=0.002 n=6)
APIResponse/[]ExemplarQueryResult-14   351.7n ± 1%    955.2n ± 1%  +171.65% (p=0.002 n=6)   450.0n ±  1%  +27.97% (p=0.002 n=6)
APIResponse/floats-10-14               3.875µ ± 2%   10.806µ ± 1%  +178.90% (p=0.002 n=6)   5.447µ ±  4%  +40.59% (p=0.002 n=6)
APIResponse/histograms-10-14           41.61µ ± 3%   143.34µ ± 0%  +244.52% (p=0.002 n=6)   49.72µ ±  1%  +19.50% (p=0.002 n=6)
APIResponse/floats-100-14              280.0µ ± 2%    790.3µ ± 1%  +182.22% (p=0.002 n=6)   423.2µ ±  2%  +51.14% (p=0.002 n=6)
APIResponse/histograms-100-14          4.000m ± 1%   14.083m ± 1%  +252.05% (p=0.002 n=6)   4.805m ±  2%  +20.12% (p=0.002 n=6)
APIResponse/floats-1000-14             27.73m ± 2%    77.84m ± 1%  +180.70% (p=0.002 n=6)   41.97m ±  3%  +51.34% (p=0.002 n=6)
APIResponse/histograms-1000-14         399.9m ± 2%   1424.6m ± 1%  +256.21% (p=0.002 n=6)   485.5m ±  4%  +21.39% (p=0.002 n=6)
geomean                                3.126µ         9.016µ       +188.41%                 4.238µ        +35.58%

                                     │     main     │               go1.26.6               │              go1.27.1               │
                                     │     B/op     │     B/op      vs base                │     B/op      vs base               │
APIResponse/AlertsResult-14              416.0 ± 0%     624.0 ± 0%   +50.00% (p=0.002 n=6)     288.0 ± 0%  -30.77% (p=0.002 n=6)
APIResponse/AlertManagersResult-14       360.0 ± 0%     512.0 ± 0%   +42.22% (p=0.002 n=6)     240.0 ± 0%  -33.33% (p=0.002 n=6)
APIResponse/ConfigResult-14              192.0 ± 0%     400.0 ± 0%  +108.33% (p=0.002 n=6)     160.0 ± 0%  -16.67% (p=0.002 n=6)
APIResponse/FlagsResult-14               192.0 ± 0%     384.0 ± 0%  +100.00% (p=0.002 n=6)     144.0 ± 0%  -25.00% (p=0.002 n=6)
APIResponse/BuildinfoResult-14           384.0 ± 0%     496.0 ± 0%   +29.17% (p=0.002 n=6)     256.0 ± 0%  -33.33% (p=0.002 n=6)
APIResponse/RuntimeinfoResult-14         536.0 ± 0%     592.0 ± 0%   +10.45% (p=0.002 n=6)     352.0 ± 0%  -34.33% (p=0.002 n=6)
APIResponse/model.LabelValues-14         192.0 ± 0%     392.0 ± 0%  +104.17% (p=0.002 n=6)     152.0 ± 0%  -20.83% (p=0.002 n=6)
APIResponse/SnapshotResult-14            192.0 ± 0%     384.0 ± 0%  +100.00% (p=0.002 n=6)     144.0 ± 0%  -25.00% (p=0.002 n=6)
APIResponse/TargetsResult-14             288.0 ± 0%     448.0 ± 0%   +55.56% (p=0.002 n=6)     176.0 ± 0%  -38.89% (p=0.002 n=6)
APIResponse/[]MetricMetadata-14          320.0 ± 0%     496.0 ± 0%   +55.00% (p=0.002 n=6)     224.0 ± 0%  -30.00% (p=0.002 n=6)
APIResponse/map[string][]Metadata-14     296.0 ± 0%     480.0 ± 0%   +62.16% (p=0.002 n=6)     208.0 ± 0%  -29.73% (p=0.002 n=6)
APIResponse/TSDBResult-14                784.0 ± 0%     752.0 ± 0%    -4.08% (p=0.002 n=6)     480.0 ± 0%  -38.78% (p=0.002 n=6)
APIResponse/TSDBBlocksResult-14          528.0 ± 0%     672.0 ± 0%   +27.27% (p=0.002 n=6)     336.0 ± 0%  -36.36% (p=0.002 n=6)
APIResponse/WalReplayStatus-14           272.0 ± 0%     416.0 ± 0%   +52.94% (p=0.002 n=6)     176.0 ± 0%  -35.29% (p=0.002 n=6)
APIResponse/[]ExemplarQueryResult-14     368.0 ± 0%     576.0 ± 0%   +56.52% (p=0.002 n=6)     240.0 ± 0%  -34.78% (p=0.002 n=6)
APIResponse/floats-10-14               3.500Ki ± 0%   3.453Ki ± 0%    -1.34% (p=0.002 n=6)   3.125Ki ± 0%  -10.71% (p=0.002 n=6)
APIResponse/histograms-10-14           42.06Ki ± 0%   40.45Ki ± 0%    -3.83% (p=0.002 n=6)   40.12Ki ± 0%   -4.61% (p=0.002 n=6)
APIResponse/floats-100-14              227.3Ki ± 0%   224.5Ki ± 0%    -1.26% (p=0.002 n=6)   224.1Ki ± 0%   -1.40% (p=0.002 n=6)
APIResponse/histograms-100-14          3.476Mi ± 0%   3.321Mi ± 0%    -4.47% (p=0.002 n=6)   3.320Mi ± 0%   -4.48% (p=0.002 n=6)
APIResponse/floats-1000-14             21.93Mi ± 0%   21.90Mi ± 0%    -0.14% (p=0.002 n=6)   21.90Mi ± 0%   -0.14% (p=0.002 n=6)
APIResponse/histograms-1000-14         353.1Mi ± 0%   337.9Mi ± 0%    -4.33% (p=0.002 n=6)   337.9Mi ± 0%   -4.33% (p=0.002 n=6)
geomean                                3.164Ki        4.257Ki        +34.51%                 2.394Ki       -24.36%

                                     │       main       │               go1.26.6                │              go1.27.1               │
                                     │    allocs/op     │  allocs/op   vs base                  │  allocs/op   vs base                │
APIResponse/AlertsResult-14                 16.000 ± 0%   10.000 ± 0%   -37.50% (p=0.002 n=6)     2.000 ±  0%   -87.50% (p=0.002 n=6)
APIResponse/AlertManagersResult-14          12.000 ± 0%    9.000 ± 0%   -25.00% (p=0.002 n=6)     2.000 ±  0%   -83.33% (p=0.002 n=6)
APIResponse/ConfigResult-14                  8.000 ± 0%    8.000 ± 0%         ~ (p=1.000 n=6) ¹   2.000 ±  0%   -75.00% (p=0.002 n=6)
APIResponse/FlagsResult-14                   8.000 ± 0%    8.000 ± 0%         ~ (p=1.000 n=6) ¹   2.000 ±  0%   -75.00% (p=0.002 n=6)
APIResponse/BuildinfoResult-14              14.000 ± 0%    8.000 ± 0%   -42.86% (p=0.002 n=6)     2.000 ±  0%   -85.71% (p=0.002 n=6)
APIResponse/RuntimeinfoResult-14            18.000 ± 0%    8.000 ± 0%   -55.56% (p=0.002 n=6)     2.000 ±  0%   -88.89% (p=0.002 n=6)
APIResponse/model.LabelValues-14             7.000 ± 0%    8.000 ± 0%   +14.29% (p=0.002 n=6)     2.000 ±  0%   -71.43% (p=0.002 n=6)
APIResponse/SnapshotResult-14                8.000 ± 0%    8.000 ± 0%         ~ (p=1.000 n=6) ¹   2.000 ±  0%   -75.00% (p=0.002 n=6)
APIResponse/TargetsResult-14                10.000 ± 0%    9.000 ± 0%   -10.00% (p=0.002 n=6)     2.000 ±  0%   -80.00% (p=0.002 n=6)
APIResponse/[]MetricMetadata-14             14.000 ± 0%    9.000 ± 0%   -35.71% (p=0.002 n=6)     2.000 ±  0%   -85.71% (p=0.002 n=6)
APIResponse/map[string][]Metadata-14        12.000 ± 0%    9.000 ± 0%   -25.00% (p=0.002 n=6)     2.000 ±  0%   -83.33% (p=0.002 n=6)
APIResponse/TSDBResult-14                   26.000 ± 0%    9.000 ± 0%   -65.38% (p=0.002 n=6)     2.000 ±  0%   -92.31% (p=0.002 n=6)
APIResponse/TSDBBlocksResult-14             22.000 ± 0%   10.000 ± 0%   -54.55% (p=0.002 n=6)     2.000 ±  0%   -90.91% (p=0.002 n=6)
APIResponse/WalReplayStatus-14              11.000 ± 0%    8.000 ± 0%   -27.27% (p=0.002 n=6)     2.000 ±  0%   -81.82% (p=0.002 n=6)
APIResponse/[]ExemplarQueryResult-14        15.000 ± 0%   10.000 ± 0%   -33.33% (p=0.002 n=6)     2.000 ±  0%   -86.67% (p=0.002 n=6)
APIResponse/floats-10-14                    48.000 ± 0%   10.000 ± 0%   -79.17% (p=0.002 n=6)     2.000 ±  0%   -95.83% (p=0.002 n=6)
APIResponse/histograms-10-14               348.000 ± 0%   10.000 ± 0%   -97.13% (p=0.002 n=6)     2.000 ±  0%   -99.43% (p=0.002 n=6)
APIResponse/floats-100-14                  408.000 ± 0%   10.000 ± 0%   -97.55% (p=0.002 n=6)     2.000 ±  0%   -99.51% (p=0.002 n=6)
APIResponse/histograms-100-14            30408.000 ± 0%   10.000 ± 0%   -99.97% (p=0.002 n=6)     2.000 ±  0%   -99.99% (p=0.002 n=6)
APIResponse/floats-1000-14                4008.000 ± 0%   10.000 ± 0%   -99.75% (p=0.002 n=6)     2.000 ±  0%   -99.95% (p=0.002 n=6)
APIResponse/histograms-1000-14         3004009.000 ± 0%   10.000 ± 0%  -100.00% (p=0.002 n=6)     4.000 ± 50%  -100.00% (p=0.002 n=6)
geomean                                      63.35         9.053        -85.71%                   2.067         -96.74%
¹ all samples are equal

[Karthik-Chowdary]

I reviewed the envelope-only decoder switch separately from the typed result decoding. Keeping Data as encoding/json.RawMessage preserves the raw payload for the existing second-stage decoders, so this does not broaden the behavior change into response models. The updated malformed-envelope expectation captures the one observable error-text change. The benchmark matrix also makes the tradeoff explicit: Go 1.26 pays CPU cost while allocations improve, and Go 1.27 substantially narrows that CPU gap. Given that this is scoped to the small response envelope and is part of the tracked jsoniter removal, the change looks sound.

[bwplotka]

Thanks

Histograms are fun (alloc/op) (yet 3x slower)

APIResponse/histograms-1000-14 3004009.000 ± 0% 10.000 ± 0% -100.00% (p=0.002 n=6) 4.000 ± 50% -100.00% (p=0.002 n=6)

[liggitt]

rebased