Support BigQuery Labels

[kconvey]

Describe the feature

Allow for tables to be configured in dbt_project.yml with BigQuery labels. Ex.

models:
  fishtown_analytics:
    labels: {town: 'fish', analytics: 'yes'}
  
    events:
      labels:
       town: 'less_fish'
       analytics: ''

Implementing this should look very similar to kms key encryption in #1829

Describe alternatives you've considered

It may be possible to do this as a post-processing step outside of dbt or post-hook macro with some more adapter functionality implemented.

Additional context

Labels can be specified in the 'OPTIONS' parameter of 'create or replace' ddl like OPTIONS(labels=[('town': 'fish')]). 'OPTIONS' requires the labels to be in the form 'ARRAY<STRUCT<STRING, STRING>>', which associates label keys with their values, making a dictionary the logical form for configuring this in dbt_project.yml.

One quirk with needing to configure this as a dictionary is that the project parser seems to think that 'labels' is a model without a matching sql file and will give the warning:
WARNING: Configuration paths exist in your dbt_project.yml file which do not apply to any resources. There are 1 unused configuration paths: - models.labels
which will not prevent this from working in most cases, but can be confusing. It may be necessary to tweak the project parser to avoid this (could use some guidance on the best way to go about this).

Who will this benefit?

Anyone who uses BigQuery and may need to have labels for their models.

[drewbanin]

Less fish? I don't buy it :)

Your thinking here is spot-on -- we can add labels as a known config for BQ here: https://github.com/fishtown-analytics/dbt/blob/e51c942e91a94936f68f2965963d3b46f1257658/plugins/bigquery/dbt/adapters/bigquery/impl.py#L51-L52

This should avoid the warning you noted above.

I'm going to close this one as a dupe of #1942 but I am very interested in supporting this in the near future!