log/slog: use a pooled json encoder

log/slog: use a pooled json encoder

goos: linux
goarch: amd64
pkg: log/slog/internal/benchmarks
cpu: 12th Gen Intel(R) Core(TM) i7-1260P
                                 │   old.txt   │               new.txt               │
                                 │   sec/op    │   sec/op     vs base                │
Attrs/JSON_discard/5_args-16       217.4n ± 8%   217.7n ± 5%        ~ (p=0.971 n=10)
Attrs/JSON_discard/5_args_ctx-16   217.8n ± 8%   219.0n ± 6%        ~ (p=0.723 n=10)
Attrs/JSON_discard/10_args-16      482.8n ± 4%   418.5n ± 1%  -13.33% (p=0.000 n=10)
Attrs/JSON_discard/40_args-16      1.709µ ± 1%   1.413µ ± 1%  -17.33% (p=0.000 n=10)
geomean                            444.5n        409.7n        -7.84%

                                 │    old.txt     │                new.txt                 │
                                 │      B/op      │     B/op      vs base                  │
Attrs/JSON_discard/5_args-16         0.000 ± 0%       0.000 ± 0%        ~ (p=1.000 n=10) ¹
Attrs/JSON_discard/5_args_ctx-16     0.000 ± 0%       0.000 ± 0%        ~ (p=1.000 n=10) ¹
Attrs/JSON_discard/10_args-16        498.0 ± 0%       273.0 ± 0%  -45.18% (p=0.000 n=10)
Attrs/JSON_discard/40_args-16      2.383Ki ± 0%     1.614Ki ± 0%  -32.25% (p=0.000 n=10)
geomean                                         ²                 -21.94%                ²
¹ all samples are equal
² summaries must be >0 to compute geomean

                                 │    old.txt    │               new.txt                │
                                 │   allocs/op   │ allocs/op   vs base                  │
Attrs/JSON_discard/5_args-16        0.000 ± 0%     0.000 ± 0%        ~ (p=1.000 n=10) ¹
Attrs/JSON_discard/5_args_ctx-16    0.000 ± 0%     0.000 ± 0%        ~ (p=1.000 n=10) ¹
Attrs/JSON_discard/10_args-16       7.000 ± 0%     3.000 ± 0%  -57.14% (p=0.000 n=10)
Attrs/JSON_discard/40_args-16      22.000 ± 0%     8.000 ± 0%  -63.64% (p=0.000 n=10)
geomean                                        ²               -37.17%                ²
¹ all samples are equal
² summaries must be >0 to compute geomean

Fixes #74645

Change-Id: I185667cd6caa6a8251e75233eb659c1fd449deaa
Reviewed-on: https://go-review.googlesource.com/c/go/+/705555
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>