feat: add Pretty exporter

[loks0n]

Summary

• Add Exporter\Pretty for colourful, human-readable multi-line span output designed for local development
• Attributes are displayed with padded keys for aligned values, duration is colour-coded (green/yellow/red), and errors are highlighted in red
• Add tests and update README with docs and usage example

Test plan

• All existing tests pass (vendor/bin/phpunit)
• New PrettyTest covers basic output, all attribute types, error handling, and span metadata

🤖 Generated with Claude Code

Summary by CodeRabbit

Release Notes

• New Features

  • Introduced Pretty exporter providing colorful, multi-line span output with color-coded durations (green/yellow/red based on severity), properly aligned attributes, comprehensive error details including exception information and stack traces, and configurable parameters for trace depth and line width.

• Documentation

  • Added Pretty exporter documentation including parameter descriptions, feature overview, and sample formatted output.

[coderabbitai]

Caution

Review failed

The pull request is closed.

Walkthrough

This pull request introduces a new Pretty exporter for rendering span data in colorful, multi-line formatted output. The changes include a new Pretty class implementing the Exporter interface that formats span attributes with alignment and color coding, outputs errors to STDERR while using STDOUT for non-errors, filters span.\* prefixed attributes, and renders truncated stack traces. Supporting additions include a test suite validating core functionality and documentation updates in README.md describing the new exporter's capabilities.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feat-pretty-exporter

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