benchmark: add micro-benchmarks for TraceID hex formatting and parsing

[AneesPatel]

Description

Adds benchmark/trace_id_bench.cpp to introduce targeted Google Benchmark coverage for the 128-bit and 64-bit TraceID formatting paths. Also updates benchmark/CMakeLists.txt to include the new benchmark executable.

Baseline Metrics (Ubuntu 22.04 / GCC 11.4):

Benchmark	Time (ns/op)	Iterations	Notes
BM_TraceID_HexPadded	204 ns	3.6M	Full 128-bit ID to 32-char hex. Cost reflects two underlying std::string allocations and concatenation.
BM_TraceID_ParseHex_128bit	108 ns	6.5M	W3C traceparent extraction path.
BM_TraceID_ParseHex_64bit	60 ns	11.8M	Legacy 64-bit extraction path.
BM_HexPadded_uint64	46 ns	17.6M	Single to_chars + std::string alloc.
BM_Hex_uint64	24 ns	29.5M	No padding.

Motivation

TraceID::hex_padded() is called on every outgoing W3C traceparent header injection, and TraceID::parse_hex() on every incoming extraction. Because these functions sit in the hottest path of distributed tracing, their latency and allocation overhead multiply across millions of spans.

Currently, the underlying hex_padded<uint64_t>() and hex<uint64_t>() functions utilize a stack-buffer-to-std::string pattern. While the existing BM_TraceTinyCCSource benchmark provides excellent end-to-end throughput data, it cannot isolate micro-regressions in these specific serialization operations.

This baseline provides the data needed to safely prevent future regressions and evaluate allocation-reduction strategies in the critical path.

Jira ticket

N/A

Additional Notes

• Validated locally via Linux (WSL2 Ubuntu 22.04) with GCC 11.4.

How to run

mkdir .build && cd .build
cmake .. -DDD_TRACE_BUILD_BENCHMARK=1 -DCMAKE_BUILD_TYPE=Release
make -j
./benchmark/dd_trace_cpp-benchmark --benchmark_filter='^BM_TraceID|^BM_Hex'