Implement OTLP protobuf encoder

[brickfrog]

Summary

• spike-gated moonbitlang/protobuf viability and resolved latest moonbitlang/protobuf@0.1.1
• implement ProtobufEncoder for OTLP traces/logs using official protobuf Writer primitives and hand-rolled OTLP field mappings
• add structural protobuf decode tests plus a Python-protobuf-derived golden trace request fixture
• update docs/otlp-protobuf-design.md with the implemented approach and runtime version

TDD

• intent: Encode OTLP ExportTraceServiceRequest/ExportLogsServiceRequest to protobuf bytes behind the ProtobufEncoder seam, with bytes ids, fixed64 timestamps, fixed32 flags, and round-trip/golden verification
• red: temporarily stubbed ProtobufEncoder.encode_spans to return an empty protobuf body; moon test --target native failed with the protobuf encoder tests
• green/done: restored implementation; moon test --target native passed (279 tests)

Wire-format audit note

• moonbitlang/protobuf resolved to 0.1.1, the latest version available in the local mooncakes registry index
• wire-format correctness is verified by structural round-trip decode tests and a GOLDEN byte fixture for a representative ExportTraceServiceRequest produced by a Python google.protobuf 7.34.1 dynamic descriptor for the same OTLP subset

Verification

• moon fmt --check
• moon test --target native
• moon build --target native --release
• moon check --target all src/otlp/transport
• moon info; moon fmt left no extra changes

No dependency on ryota0624/opentelemetry_proto_mbt or protoc-gen-mbt.

Verification

Generated by Choir from commands executed in the leaf workspace.

• moon fmt --check
  • exit: 0
  • head: 7786caa
  • output tail:

Finished. moon: no work to do

• moon test --target native
  • exit: 0
  • head: 7786caa
  • output tail:

Total tests: 279, passed: 279, failed: 0.

• moon build --target native --release
  • exit: 0
  • head: 7786caa
  • output tail:

Finished. moon: no work to do

[brickfrog]

Choir: GitHub Actions rollup reports all checks successful on the latest head — re-check review/merge policy on GitHub (not an approval or merge signal by itself).

[Copilot]

Pull request overview

This PR adds OTLP protobuf encoding support to the src/otlp/transport layer by implementing a ProtobufEncoder that emits ExportTraceServiceRequest / ExportLogsServiceRequest protobuf bytes using moonbitlang/protobuf, along with round-trip structural decoder tests and a golden-byte fixture to validate wire-format correctness.

Changes:

• Add moonbitlang/protobuf@0.1.1 as a dependency and import it in the OTLP transport package.
• Implement OTLP trace/log protobuf encoding (field mappings + primitive writer helpers) and introduce EncodingError::ProtobufEncodeFailed for structured failures.
• Add extensive protobuf decode-based tests plus a golden bytes assertion for a representative trace request; update protobuf design documentation accordingly.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/otlp/transport/encoding.mbt	Implements OTLP protobuf writer helpers, trace/log request encoders, and a new structured encoding error.
src/otlp/transport/encoding_test.mbt	Adds structural protobuf decoders + assertions for traces/logs, a golden bytes test, and error-path validation.
src/otlp/transport/moon.pkg	Imports moonbitlang/protobuf so transport code/tests can use protobuf writer/reader APIs.
moon.mod	Adds moonbitlang/protobuf@0.1.1 dependency.
src/otlp/transport/pkg.generated.mbti	Updates the public interface to include the new ProtobufEncodeFailed error variant.
docs/otlp-protobuf-design.md	Updates the design doc to reflect the implemented hand-rolled OTLP subset approach and runtime version.

[brickfrog]

Choir: GitHub Copilot issue-thread comment observed on PR — parent notified with updated gate snapshot (not merge approval).