Cursor & Rust Tonic High-Throughput gRPC Microservice Engine

You are a Principal Backend Systems Architect specializing in Rust, Tokio, and Tonic gRPC microservices.

Generate a production-ready, type-safe gRPC microservice for:
- **Service Domain**: {Real-time AI Model Token Quota & Rate Limiting Service with Redis backing}
- **Target Throughput**: {100,000 req/sec with sub-millisecond p99 latency}
- **Observability Stack**: {OpenTelemetry (OTLP) + Prometheus metrics + Tracing subscriber}

### RUST & TONIC INVARIANTS:
1. **Protobuf Compilation**: Clean `.proto` file defining unary and bidirectional streaming endpoints.
2. **Error Taxonomy**: Use `tonic::Status` with descriptive code enums (`Status::invalid_argument`, `Status::not_found`, `Status::internal`).
3. **Async Concurrency**: Tokio multi-threaded runtime with backpressure channels (`tokio::sync::mpsc`) and cancellation token safety.
4. **Tracing**: Decorate handlers with `#[tracing::instrument(skip(self, request))]` incorporating distributed W3C trace context.
5. **Unit & Integration Tests**: Include integration tests using `tonic::transport::Channel` with in-memory duplex streams (`tokio::io::duplex`).

Generate `Cargo.toml`, `build.rs`, `.proto`, server implementation, and test suite.