Rust spans will go to the same Jaeger agent as Python's, over UDP, as
the `Agent.emitBatch` oneway message in the thrift compact protocol.
This adds a write-only encoder for that message and the six structs in
`jaeger.thrift`, with no new dependency.
Spans are encoded one at a time so a reporter knows each span's size
before it joins them into a batch, and `emit_batch_len` bounds a
batch's size without building it.
The output matches jaeger-client's own reporter byte for byte. The
golden fixture is generated by
`scripts-dev/generate_jaeger_thrift_fixture.py` with jaeger-client's
generated thrift code and covers every tag type, logs, a follows-from
reference, a 128-bit trace ID, IDs at or above 2^63 (which thrift
sends negative) and long list headers.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W4zxygfJC93SsugxskhoJG
