Docs (talks about datasets, but tables can have labels, too):
https://cloud.google.com/bigquery/docs/labeling-datasets

API references:
https://cloud.google.com/bigquery/docs/reference/rest/v2/datasets#labels
https://cloud.google.com/bigquery/docs/reference/rest/v2/tables#labels

If the patch request is not yet supported, this will be needed, too.
https://cloud.google.com/bigquery/docs/reference/rest/v2/datasets/patch
https://cloud.google.com/bigquery/docs/reference/rest/v2/tables/patch