fix(google-genai): propagate context to threaded tool calls (#38)

[RichardoMrMu]

What

Fixes #38. Parallel/concurrent tool calls in an agent produce multiple
disconnected traces instead of one trace with multiple spans.

Root cause

tool_call_wrapper.wrapped_tool() wraps each tool while the agent/LLM span is
active (generate_content._wrapped_config_with_tools). The wrapped tool builds
its ToolInvocation -> start_execute_tool, whose span is parented to whatever
is in the OpenTelemetry context at call time. The Google GenAI SDK's
automatic function calling (and agent frameworks) execute those wrapped tools in
a ThreadPoolExecutor / run_in_executor worker. Worker threads do not inherit
contextvars, so the worker sees an empty context and each tool span starts a
new root trace.

This matches the maintainer's confirmed analysis on #38, and is the same class
of issue already worked around per-instrumentation in bfclv4
(threading_propagation.py) and openhands (session_context.py).

Fix

tool_call_wrapper now captures the active context when a tool is wrapped
(only when a span is active) and re-attaches it around invocation creation in
both the sync and async wrappers, then detaches immediately. The tool function
itself runs in its normal context; only the invocation/span creation is
re-parented. When no span is active the capture returns None and behavior is
unchanged, so single-threaded execution is unaffected.

Only tool_call_wrapper.py changes on the source side; _compat.py and
generate_content.py are untouched.

Tests

Added two regression tests in tests/utils/test_tool_call_wrapper.py:

• test_parallel_tool_calls_share_parent_trace runs two wrapped tools in a
  ThreadPoolExecutor under an agent span and asserts both execute_tool spans
  share the agent's trace_id.
• test_run_in_executor_tool_call_shares_parent_trace covers the
  asyncio.run_in_executor path named in the issue.

Both fail on the current wrapper (tool spans land on new traces) and pass with
the fix. The existing single-threaded tool-wrapper tests are unchanged and still
pass.

[Copilot]

Copilot review overview

🟡 Changes recommended

Out-of-order context-token detachment leaks the captured trace context into reused workers.

Get a fresh assessment by requesting another Copilot review.

Review effort: Balanced
Findings: 1

Open (1)

• Detach captured parent token after invocation completes · New

What changed in this PR

Propagates OpenTelemetry context to threaded Google GenAI tool calls.

Changes:

• Captures and reattaches parent context for tool spans.
• Adds thread-pool regression tests.
• Documents the fix.

File	Description
tool_call_wrapper.py	Adds context propagation.
test_tool_call_wrapper.py	Tests threaded tool calls.
CHANGELOG.md	Records the fix.

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