feat: add user-agent delegation telemetry for pandas-gbq

[shuoweil]

Appends pandas-gbq/<version> to ClientInfo.user_agent when RowIterator.to_dataframe() or RowIterator.to_geodataframe() is executed and query delegation is supported by pandas-gbq.

This PR enables backend metrics and BigQuery audit logs to differentiate between direct client-side conversion and delegated conversions, without altering existing DataFrame results or runtime behavior.

Supersedes #17704.

Key Changes

• google/cloud/bigquery/table.py: Injects pandas-gbq/<version> into client_info.user_agent with deduplication check when PANDAS_GBQ_VERSIONS.is_delegation_supported is True.
• tests/unit/test_table.py: Adds unit tests for user-agent injection, deduplication, missing client_info/user_agent, and unsupported fallback behavior.

Fixes #<540939659> 🦕

[gemini-code-assist]

Code Review

This pull request updates the to_dataframe method in table.py to append pandas-gbq telemetry to the client's user agent when delegation is supported, and adds comprehensive unit tests to verify this behavior. The review feedback suggests wrapping the user-agent update logic in a try-except block to ensure robust defensive programming and prevent potential crashes. Additionally, it recommends mocking installed_version in the geodataframe tests to avoid import errors in environments where pandas-gbq is not installed.

[gemini-code-assist]

Telemetry and user-agent updates should be treated as best-effort operations. If client_info is a mock, a custom read-only object, or lacks the user_agent attribute, accessing or modifying client_info.user_agent directly could raise an AttributeError or other exceptions, causing the core to_dataframe() call to crash.

To ensure robust defensive programming, we should wrap this telemetry update in a try-except block and use getattr to safely access user_agent. Avoid silently passing on exceptions; instead, log the exception using logger.warning to aid in debugging and prevent masking underlying issues.

        if _versions_helpers.PANDAS_GBQ_VERSIONS.is_delegation_supported:
            try:
                client_info = getattr(
                    getattr(self.client, "_connection", None), "_client_info", None
                )
                if client_info:
                    ua = getattr(client_info, "user_agent", None) or ""
                    if "pandas-gbq" not in ua:
                        version = _versions_helpers.PANDAS_GBQ_VERSIONS.installed_version
                        client_info.user_agent = f"{ua} pandas-gbq/{version}".strip()
            except Exception as exc:
                logger.warning("Failed to update telemetry user-agent: %s", exc)

References

1. Avoid broad except Exception: blocks that silently return None. Instead, log the exception (e.g., using logger.warning) to aid in debugging and prevent masking underlying issues.

[shuoweil]

fixed

[sycai]

Do we need this logging? I think it would print unwanted contents on the user's terminal or notebook, right?

[sycai]

Could you share the link to the code where pandas-gbq is used as the delegation?