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

❌ Patch coverage is 81.48148% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.03%. Comparing base (fa603ec) to head (3895d3f).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/xds/resolver/serviceconfig.go	28.57%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9297      +/-   ##
==========================================
- Coverage   83.06%   83.03%   -0.03%     
==========================================
  Files         423      423              
  Lines       35215    35224       +9     
==========================================
- Hits        29251    29250       -1     
- Misses       4440     4446       +6     
- Partials     1524     1528       +4     

Files with missing lines	Coverage Δ
internal/transport/http2_server.go	91.71% <100.00%> (+1.28%)	⬆️
internal/xds/server/routing.go	84.41% <100.00%> (-0.59%)	⬇️
internal/xds/xdsclient/xdsresource/matcher.go	69.90% <100.00%> (-2.17%)	⬇️
internal/xds/resolver/serviceconfig.go	88.18% <28.57%> (-3.55%)	⬇️

... and 23 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.