add user-agent "dbt-{version}" to the BigQuery connection

[fhoffa]

Describe the feature

Add an user-agent "dbt-{version}" to the BigQuery connection.

Who will this benefit?

Google can keep track of the most popular libraries used with BigQuery - and then use this information to prioritize features and recognize the most useful tools. This is done by populating the user-agent string with the corresponding "application name" and version information.

Reference

This is a similar issue and code:

• BigQuery: Add user-agent info to Jupyter magics Client constructor(s). googleapis/google-cloud-python#8696

The API definition for ClientInfo.user_agent:

• https://googleapis.dev/python/bigquery/latest/generated/google.cloud.bigquery.client.Client.html

The client info used to send a user-agent string along with API requests. If None, then default info will be used. Generally, you only need to set this if you’re developing your own library or partner tool.

Where the change would be applied:

• At "google.cloud.bigquery.Client()" - https://github.com/fishtown-analytics/dbt/blob/dev/barbara-gittings/plugins/bigquery/dbt/adapters/bigquery/connections.py#L141

[drewbanin]

Thanks @fhoffa - didn't know about that setting. It's a great idea. I just prioritized this one for our next release (v0.16.0 - Barbara Gittings) :)

[tswast]

Example of the feature in use to track %%bigquery IPython magics usage: https://github.com/googleapis/python-bigquery/blob/3d77a75c86a599f7d36e68fea7979d748819e875/google/cloud/bigquery/magics.py#L491