Add span functions

[tgillam-do]

Adds span functions for capturing spans manually without using the decorators. Provides a more robust approach to capturing spans.

Adds to README.md and adds integrations tests that ensures agents can respond and run appropriately when using these new functions.

[Copilot]

Pull request overview

This PR adds programmatic span functions (add_llm_span, add_tool_span, add_agent_span) to provide manual span creation capabilities as an alternative to the existing decorators.

Key Changes:

• New programmatic span functions for creating LLM, tool, and agent spans without decorators
• Support for comprehensive metadata including token counts, temperature, duration, and custom fields
• Integration with existing DigitalOcean tracker to serialize new span types

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
gradient_adk/tracing.py	Implements three new programmatic span functions with detailed metadata support
gradient_adk/init.py	Exports new span functions in public API
gradient_adk/runtime/digitalocean_tracker.py	Adds serialization logic for agent and tool span types created programmatically
tests/tracing_test.py	Comprehensive unit tests covering all span functions and edge cases
integration_tests/programmatic_spans/test_programmatic_spans.py	Integration tests verifying end-to-end functionality with test agent
integration_tests/example_agents/programmatic_spans_agent/main.py	Example agent demonstrating usage of all three span functions
README.md	Updated documentation with examples and comparison of decorators vs programmatic functions

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