Add GitHub Copilot repository instructions with development workflow guidance

.github/copilot-instructions.md

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+# OpenTelemetry Rust Copilot Instructions
+
+This repository contains the OpenTelemetry implementation for Rust. When working with this codebase, please follow these guidelines:
+
+## Development Tools and Commands
+
+1. **Use clippy, rustfmt, and cargo check regularly.**
+   - Run `cargo clippy --workspace --all-targets --all-features` for comprehensive linting
+   - Run `cargo fmt --all` to format code consistently
+   - Run `cargo check --workspace` to verify compilation
+
+2. **Use the provided development scripts:**
+   - Run `./scripts/precommit.sh` before committing (runs format, lint, and test)
+   - Run `./scripts/lint.sh` for comprehensive linting across all features
+   - Run `./scripts/test.sh` for running tests
+
+## Workspace Structure
+
+This is a Cargo workspace containing multiple crates:
+- `opentelemetry` - Core API crate
+- `opentelemetry-sdk` - SDK implementation
+- `opentelemetry-otlp` - OTLP exporter
+- `opentelemetry-stdout` - Stdout exporter
+- And many more specialized crates
+
+When making changes, consider the impact across the entire workspace.
+
+## Feature Flags and Compilation
+
+- This codebase uses extensive feature flags for signals (`traces`, `metrics`, `logs`)
+- Experimental features are behind the `otel_unstable` feature flag
+- Use `cargo hack --each-feature` to test across feature combinations
+- Some crates like `opentelemetry-prometheus` are outside the main workspace
+
+## Code Standards
+
+- **MSRV**: Minimum Supported Rust Version is 1.75
+- **Error Handling**: Wrap errors in signal-specific types (`TraceError`, `MetricError`, `LogsError`)
+- **Commits**: Use conventional commit format (e.g., `feat:`, `fix:`, `docs:`)
+- **Testing**: Run `cargo test --workspace` for comprehensive testing
+- **Documentation**: Include examples in documentation and maintain README files
+
+## OpenTelemetry Specifics
+
+- Follow the OpenTelemetry specification
+- Prioritize functionality over structural compliance with spec
+- Use Rust idioms rather than forcing spec API patterns
+- Configuration priority: compile-time config > Environment variables
+- Use `#[cfg(feature = "otel_unstable")]` for experimental features
+
+## Build Requirements
+
+- For `opentelemetry-otlp`: Requires `protoc` (Protocol Buffers compiler)
+- Clone with submodules: `git clone --recurse-submodule`
+- Set `PROTOC` environment variable if needed: `export PROTOC=$(which protoc)`
+
+## Testing and Validation
+
+- Run benchmarks with `cargo bench` for performance-sensitive changes
+- Test examples in the `examples/` directory to ensure they work
+- Validate changes don't break existing behavior across workspace
+- Consider impact on different runtime features (`rt-tokio`, etc.)