transport: avoid slice reallocation during header creation

[arjan-bal]

This PR improves the size estimate while pre-allocating headerFields to avoid reallocations, which pprof showed were responsible for ~4% of total memory allocations. This change improves performance, increasing QPS by 1% while reducing bytes/op by 4% and latencies by 0.3-4%.

Tested

go run benchmark/benchresult/main.go unary-before unary-after       
unary-networkMode_Local-bufConn_false-keepalive_false-benchTime_1m0s-trace_false-latency_0s-kbps_0-MTU_0-maxConcurrentCalls_120-reqSiz
e_1024B-respSize_1024B-compressor_off-channelz_false-preloader_false-clientReadBufferSize_-1-clientWriteBufferSize_-1-serverReadBuffer
Size_-1-serverWriteBufferSize_-1-sleepBetweenRPCs_0s-connections_1-recvBufferPool_simple-sharedWriteBuffer_false
               Title       Before        After Percentage
            TotalOps      6327736      6390728     1.00%
             SendOps            0            0      NaN%
             RecvOps            0            0      NaN%
            Bytes/op     13903.23     13354.55    -3.95%
           Allocs/op       156.22       155.23    -0.64%
             ReqT/op 863946888.53 872547396.27     1.00%
            RespT/op 863946888.53 872547396.27     1.00%
            50th-Lat    1.00991ms   1.006914ms    -0.30%
            90th-Lat   1.678329ms   1.610331ms    -4.05%
            99th-Lat   2.517556ms   2.497122ms    -0.81%
             Avg-Lat   1.136117ms   1.125311ms    -0.95%
           GoVersion     go1.24.4     go1.24.4
         GrpcVersion   1.76.0-dev   1.76.0-dev

RELEASE NOTES:

• client: Improve header slice length estimate to reduce re-allocations.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.92%. Comparing base (5ed7cf6) to head (90e91b9).
⚠️ Report is 17 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8547      +/-   ##
==========================================
- Coverage   81.99%   81.92%   -0.07%     
==========================================
  Files         412      413       +1     
  Lines       40520    40541      +21     
==========================================
- Hits        33225    33215      -10     
- Misses       5904     5957      +53     
+ Partials     1391     1369      -22     

Files with missing lines	Coverage Δ
internal/transport/http2_client.go	91.99% <100.00%> (+0.17%)	⬆️

... and 48 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[easwars]

Does it make sense to also check for callHdr.SendCompress != "" and callHdr.PreviousAttempts > 0 and increment as well?

[arjan-bal]

Added these checks as well.

---

I only added the grpc-timeout header initially since it was the one showing up in the profiles while running the benchmark. This was probably because compression was disabled and retries can't be configured in the benchmark. The improvements were similar after adding checks for the two conditions mentioned above and running the benchmark again with compression enabled.