Fix bug with metrics/tracing middleware and HEAD responses

[adam-fowler]

Previously when you set the body on a response if that body had a known length it would always fix the content-length header in a didSet call. The metrics and tracing middleware do this for every response so they can alter the response body writer to update timers and spans. This breaks responses from HEAD requests as the body is set on them and it resets the content-length.

This fix changes the didSet to a set and compares the old response body with the new one. If they have the same size then we don't update the content-length header.

• If you have a response from a head request you can only set the response body to be a body with length zero, so the response body length will not change as it is passed down and any content-length header with be fixed.
• All other responses the content-length header should be consistent with the response body length so will be changed at the relevant times.

This PR also fixes an issue where if you had a response with a content-length header and you later set the response body to be of an indeterminate length (perhaps you passed the response through a streaming compression middleware) it should clear the content-length header but it wasn't

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.05%. Comparing base (e0818ae) to head (d066496).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #777      +/-   ##
==========================================
- Coverage   83.08%   83.05%   -0.03%     
==========================================
  Files         124      124              
  Lines        6178     6180       +2     
==========================================
  Hits         5133     5133              
- Misses       1045     1047       +2     

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[github-actions]

✅ Pull request no significant performance differences ✅

Summary

New baseline 'pull_request' is WITHIN the 'main' baseline thresholds.

Full Benchmark Comparison

Comparing results between 'main' and 'pull_request'

Host 'a14422710706' with 4 'x86_64' processors with 15 GB memory, running:
#18~24.04.1-Ubuntu SMP Sat Jun 28 04:46:03 UTC 2025

HummingbirdBenchmarks

HTTP:Cookie:Decode metrics

Malloc (total): results within specified thresholds, fold down for details.

Malloc (total) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	4	4	4	4	4	4	4	336
pull_request	4	4	4	4	4	4	4	333
Δ	0	0	0	0	0	0	0	-3
Improvement %	0	0	0	0	0	0	0	-3

HTTP:URI:Decode metrics

Malloc (total): results within specified thresholds, fold down for details.

Malloc (total) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	1	1	1	1	1	1	1	3418
pull_request	1	1	1	1	1	1	1	3372
Δ	0	0	0	0	0	0	0	-46
Improvement %	0	0	0	0	0	0	0	-46

HTTP:URI:QueryParameters metrics

Malloc (total): results within specified thresholds, fold down for details.

Malloc (total) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	7	7	7	7	7	7	7	885
pull_request	7	7	7	7	7	7	7	883
Δ	0	0	0	0	0	0	0	-2
Improvement %	0	0	0	0	0	0	0	-2

Router:Echo metrics

Malloc (total): results within specified thresholds, fold down for details.

Malloc (total) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	768	768	768	768	768	768	768	1092
pull_request	768	768	768	768	768	768	768	1049
Δ	0	0	0	0	0	0	0	-43
Improvement %	0	0	0	0	0	0	0	-43

Router:GET metrics

Malloc (total): results within specified thresholds, fold down for details.

Malloc (total) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	468	468	468	468	468	468	468	2909
pull_request	468	468	468	468	468	468	468	2906
Δ	0	0	0	0	0	0	0	-3
Improvement %	0	0	0	0	0	0	0	-3

Router:Middleware metrics

Malloc (total): results within specified thresholds, fold down for details.

Malloc (total) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	373	373	373	373	373	373	373	2710
pull_request	373	373	373	373	373	373	373	2612
Δ	0	0	0	0	0	0	0	-98
Improvement %	0	0	0	0	0	0	0	-98

Router:PUT metrics

Malloc (total): results within specified thresholds, fold down for details.

Malloc (total) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	1270	1270	1270	1270	1270	1270	1270	821
pull_request	1270	1270	1270	1270	1270	1270	1270	841
Δ	0	0	0	0	0	0	0	20
Improvement %	0	0	0	0	0	0	0	20

Router:Parameters metrics

Malloc (total): results within specified thresholds, fold down for details.

Malloc (total) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	668	668	668	668	668	668	668	2430
pull_request	668	668	668	668	668	668	668	2256
Δ	0	0	0	0	0	0	0	-174
Improvement %	0	0	0	0	0	0	0	-174

RouterBuilder:Middleware metrics

Malloc (total): results within specified thresholds, fold down for details.

Malloc (total) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	372	372	372	372	372	372	372	2712
pull_request	372	372	372	372	372	372	372	2655
Δ	0	0	0	0	0	0	0	-57
Improvement %	0	0	0	0	0	0	0	-57

TrieRouter:LongPaths metrics

Malloc (total): results within specified thresholds, fold down for details.

Malloc (total) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	2	2	2	2	2	2	2	231
pull_request	2	2	2	2	2	2	2	232
Δ	0	0	0	0	0	0	0	1
Improvement %	0	0	0	0	0	0	0	1

TrieRouter:Parameters metrics

Malloc (total): results within specified thresholds, fold down for details.

Malloc (total) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	7	7	7	7	7	7	7	186
pull_request	7	7	7	7	7	7	7	188
Δ	0	0	0	0	0	0	0	2
Improvement %	0	0	0	0	0	0	0	2

TrieRouter:Routing metrics

Malloc (total): results within specified thresholds, fold down for details.

Malloc (total) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	2	2	2	2	2	2	2	244
pull_request	2	2	2	2	2	2	2	247
Δ	0	0	0	0	0	0	0	3
Improvement %	0	0	0	0	0	0	0	3

URLEncodedForm:Decode metrics

Malloc (total): results within specified thresholds, fold down for details.

Malloc (total) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	29	29	29	29	29	29	29	54
pull_request	29	29	29	29	29	29	29	53
Δ	0	0	0	0	0	0	0	-1
Improvement %	0	0	0	0	0	0	0	-1

URLEncodedForm:Encode metrics

Malloc (total): results within specified thresholds, fold down for details.

Malloc (total) *	p0	p25	p50	p75	p90	p99	p100	Samples
main	24	24	24	24	24	24	24	209
pull_request	24	24	24	24	24	24	24	208
Δ	0	0	0	0	0	0	0	-1
Improvement %	0	0	0	0	0	0	0	-1