log/slog: optimize appendKey to reduce allocations

log/slog: optimize appendKey to reduce allocations

This change introduces a new method, `appendTwoStrings`, which
optimizes the `appendKey` function by avoiding the allocation of a
temporary string (string concatenation of prefix and key). Instead, it
directly appends the prefix and key to the buffer.

Additionally, added `BenchmarkAppendKey` benchmark tests to validate performance improvements.

This change improves performance in cases where large prefixes are used,
as verified by the following benchmarks:

goos: darwin
goarch: arm64
pkg: log/slog
cpu: Apple M1 Max
                             │   old.out   │               new.out               │
                             │   sec/op    │   sec/op     vs base                │
AppendKey/prefix_size_5-10     44.41n ± 0%   35.62n ± 0%  -19.80% (p=0.000 n=10)
AppendKey/prefix_size_10-10    48.17n ± 0%   39.12n ± 0%  -18.80% (p=0.000 n=10)
AppendKey/prefix_size_30-10    84.50n ± 0%   62.30n ± 0%  -26.28% (p=0.000 n=10)
AppendKey/prefix_size_50-10    124.9n ± 0%   102.3n ± 0%  -18.09% (p=0.000 n=10)
AppendKey/prefix_size_100-10   203.6n ± 1%   168.7n ± 0%  -17.14% (p=0.000 n=10)
geomean                        85.61n        68.41n       -20.09%

                             │    old.out    │                 new.out                 │
                             │     B/op      │    B/op     vs base                     │
AppendKey/prefix_size_5-10      0.000 ± 0%     0.000 ± 0%         ~ (p=1.000 n=10) ¹
AppendKey/prefix_size_10-10     0.000 ± 0%     0.000 ± 0%         ~ (p=1.000 n=10) ¹
AppendKey/prefix_size_30-10     48.00 ± 0%      0.00 ± 0%  -100.00% (p=0.000 n=10)
AppendKey/prefix_size_50-10    128.00 ± 0%     64.00 ± 0%   -50.00% (p=0.000 n=10)
AppendKey/prefix_size_100-10    224.0 ± 0%     112.0 ± 0%   -50.00% (p=0.000 n=10)
geomean                                    ²               ?                       ² ³
¹ all samples are equal
² summaries must be >0 to compute geomean
³ ratios must be >0 to compute geomean

                             │   old.out    │                 new.out                 │
                             │  allocs/op   │ allocs/op   vs base                     │
AppendKey/prefix_size_5-10     0.000 ± 0%     0.000 ± 0%         ~ (p=1.000 n=10) ¹
AppendKey/prefix_size_10-10    0.000 ± 0%     0.000 ± 0%         ~ (p=1.000 n=10) ¹
AppendKey/prefix_size_30-10    1.000 ± 0%     0.000 ± 0%  -100.00% (p=0.000 n=10)
AppendKey/prefix_size_50-10    2.000 ± 0%     1.000 ± 0%   -50.00% (p=0.000 n=10)
AppendKey/prefix_size_100-10   2.000 ± 0%     1.000 ± 0%   -50.00% (p=0.000 n=10)
geomean                                   ²               ?                       ² ³
¹ all samples are equal
² summaries must be >0 to compute geomean
³ ratios must be >0 to compute geomean

This patch improves performance without altering the external behavior of the `slog` package.

Change-Id: I8b47718de522196f06e0ddac48af73e352d2e5cd
Reviewed-on: https://go-review.googlesource.com/c/go/+/631415
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>