BigQuery already sets a user-agent: https://github.com/googleapis/python-bigquery/blob/8712b6f4f6c91b0f5aca3de98efe81deb2c7262f/google/cloud/bigquery/client.py#L4129

IIRC, this is mostly so that we can use a consistent user-agent with the rest of the client library (partner requests need attribution, for example)

#295 is causing test regressions in the BigQuery libraries.