[herin049]

Summary

The text encoders write every small fragment ("{", ",", "=\"", numbers, …) through &mut dyn Write. These small writes introduce measurable overhead due to dynamic dispatch.

This PR adds a new TextBuf type to buffer writes to the underlying writer to ensure hot path writes are statically dispatched.

Benchmarks (Apple M5 Pro - 48GB memory, rustc 1.94.1)

cargo bench --bench text, compared with --baseline against master:

OpenMetrics (encoding::text)

Benchmark	master	this PR	Change
encode	11.56 ms	9.15 ms	−20.9%
encode_realistic_string_labels	12.20 ms	9.74 ms	−20.2%
encode_escaped_label_values	14.05 ms	10.45 ms	−25.6%

Prometheus text (encoding::prometheus_text)

Benchmark	master	this PR	Change
prometheus_text_encode	11.09 ms	8.48 ms	−23.5%
prometheus_text_encode_realistic_string_labels	11.81 ms	9.18 ms	−22.3%
prometheus_text_encode_escaped_label_values	13.49 ms	9.54 ms	−29.3%

Buffer sizes from 1 to 64 KiB all landed within about 3% of each other. 4 KiB was chosen to keep the footprint small.

[krisztianfekete]

Nice, thanks, LGTM with the inline fix. Please also add CHANGELOG entries for the output changes.

[krisztianfekete]

Thanks, please sign the DCO so we can merge this!