tnewman / xk6-kafka

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perf: restore producer throughput in compat produce config decode (#415)

* perf: restore producer throughput in compat produce config decode

The protobuf serde PR (#389) replaced the produce config decode with a
mapstructure decoder using a per-field reflection hook. That double
reflection walk (export to map, then mapstructure decode) runs on every
produce() call and roughly halved writer throughput: the README
benchmark (scripts/test_json.js, 50 VUs, 60s) dropped from ~373k to
~208k msg/s. Found via git bisect.

Restore the fast JSON round-trip decode and keep the plain-string
payload fix from #389 by normalizing message key/value payloads in the
exported map first: strings and number arrays become []byte in place,
which round-trips losslessly through the base64 semantics encoding/json
applies to []byte fields. Handles both export shapes ([]any from
script-created values, []map[string]any from Go-created ones).

Benchmark on the same machine/container: ~208k -> ~372-382k msg/s,
matching the v2.0.0 tag's ~373k msg/s.

* docs: refresh benchmark results and note v2.1.0 regression

Re-measured scripts/test_json.js (50 VUs, 60s) on the reference machine:
372,484 msg/s (best observed today: 382,078 msg/s), matching the
documented v2.0.0 figure. Also fixes the stale v1.x.x/main framing and
adds a note about the v2.1.0 producer throughput regression.

23 of 40 new or added lines in 1 file covered. (57.5%)

4067 of 5190 relevant lines covered (78.36%)

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