transport: optimize header field allocation

[2307vivek]

In http2_server.go, both writeHeaderLocked and writeStatus initialized header field slices with a hard-coded capacity of 2, then appended all metadata values via appendHeaderFieldsFromMD. With metadata-heavy responses, the slice would double its capacity multiple times, each re-allocation copying all existing elements.

This pr pre-sizes header field slices to the exact count of metadata header fields before appending, eliminating repeated slice re-allocations during response header/trailer construction.

Benchmarks

$ go test ./internal/transport/ -bench='BenchmarkAppendHeaderFieldsFromMD' -benchmem -run='^$' -timeout 60s

Metadata Count	Allocs/op (Before → After)	Bytes/op (Before → After)	Time/op (Before → After)
0	0 → 0	0 → 0 B	18.0 → 26.3 ns
4	2 → 1 (-50%)	480 → 320 B (-33%)	632 → 585 ns (-7.5%)
12	4 → 1 (-75%)	2,592 → 1,024 B (-60%)	2,456 → 1,219 ns (-50%)
48	6 → 1 (-83%)	11,808 → 4,096 B (-65%)	10,765 → 5,323 ns (-51%)

$ go run benchmark/benchmain/main.go -benchtime=60s -workloads=unary -compression=off -maxConcurrentCalls=120 -trace=off -reqSizeBytes=1024 -respSizeBytes=1024 -networkMode=Local -recvBufferPool=simple -resultFile="unaryafter"
$ go run benchmark/benchresult/main.go unarybefore unaryafter

Title         Before           After            Percentage
-----         ------           -----            ----------
TotalOps      2981316          3126535          4.87%
SendOps       0                0                NaN%
RecvOps       0                0                NaN%
Bytes/op      12969.88         13010.16         0.32%
Allocs/op     132.48           132.46           0.00%
ReqT/op       407049011.20     426876245.33     4.87%
RespT/op      407049011.20     426876245.33     4.87%
50th-Lat      2.260482ms       2.158989ms      -4.49%
90th-Lat      3.573724ms       3.39454ms       -5.01%
99th-Lat      5.42035ms        5.141398ms      -5.15%
Avg-Lat       2.413806ms       2.301533ms      -4.65%
GoVersion     go1.26.5         go1.26.5
GrpcVersion   1.84.0-dev       1.84.0-dev

RELEASE NOTES: none

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: 2307vivek / name: Vivek Singh (9e20c85)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.56%. Comparing base (03255a9) to head (2dfd478).
⚠️ Report is 39 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9297      +/-   ##
==========================================
+ Coverage   83.08%   87.56%   +4.47%     
==========================================
  Files         423      429       +6     
  Lines       35210    30637    -4573     
==========================================
- Hits        29254    26826    -2428     
+ Misses       4436     3811     -625     
+ Partials     1520        0    -1520     

Files with missing lines	Coverage Δ
internal/transport/http2_client.go	94.85% <100.00%> (+2.25%)	⬆️
internal/transport/http2_server.go	93.41% <100.00%> (+3.22%)	⬆️

... and 427 files with indirect coverage changes

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[2307vivek]

Gentle ping @mbissa.

Also the failing test is not related to these changes. Would appreciate if someone can rerun the checks again.

[2307vivek]

BenchmarkServerHeaderFields 10 samples, benchstat:

mdCount	benchmark	allocs/op	B/op	sec/op
0	writeHeaderLocked	3 → 3	175 → 174 (−0.29%)	~ (indistinguishable)
0	writeStatus	6 → 5 (−16.67%)	391 → 311 (−20.46%)	−14.81% (p=0.000)
4	writeHeaderLocked	6 → 3 (−50.00%)	1367 → 510 (−62.69%)	−18.27% (p=0.000)
4	writeStatus	8 → 5 (−37.50%)	1418 → 630 (−55.56%)	−15.75% (p=0.000)
12	writeHeaderLocked	7 → 3 (−57.14%)	2708 → 1218 (−55.03%)	−13.59% (p=0.001)
12	writeStatus	9 → 5 (−44.44%)	2762 → 1271 (−53.98%)	−13.99% (p=0.000)

[2307vivek]

Just a gentle ping @mbissa