OTLP wire types, generated at build time from the protos vendored under
proto/ (upstream opentelemetry-proto v1.9.0).
We do not depend on the opentelemetry-proto crate: it declares
opentelemetry and opentelemetry_sdk as non-optional dependencies (~12
extra crates including a full SDK and rand), and its codegen does not
enable bytes = "bytes", so every trace_id would be a heap allocation.