BigQuery: Add debug logging statements to track when BQ Storage API is used.

[tswast]

With BQ Storage API and logging.basicConfig(level=logging.DEBUG):

import google.cloud.bigquery_storage_v1beta1
import google.cloud.bigquery
client = google.cloud.bigquery.Client()
bqstorage = google.cloud.bigquery_storage_v1beta1.BigQueryStorageClient()
rows = client.list_rows("schema_examples.scalar_fields")

import logging
logging.basicConfig(level=logging.DEBUG)
df = rows.to_dataframe(bqstorage_client=bqstorage)

Outputs:

DEBUG:google.auth.transport.requests:Making request: POST https://oauth2.googleapis.com/token
DEBUG:urllib3.connectionpool:Starting new HTTPS connection (1): oauth2.googleapis.com:443
DEBUG:urllib3.connectionpool:https://oauth2.googleapis.com:443 "POST /token HTTP/1.1" 200 None
DEBUG:google.cloud.bigquery._pandas_helpers:Started reading table 'swast-scratch.schema_examples.scalar_fields' with BQ Storage API session 'projects/swast-scratch/locations/us/sessions/CAISCEtpQ1dKVERqGgJpchoCaXcaAmpkGgJpbhoCamMaAmpx'.

With tabledata.list:

DEBUG:google.cloud.bigquery.table:Started reading table 'swast-scratch.schema_examples.scalar_fields' with tabledata.list.
DEBUG:urllib3.connectionpool:https://www.googleapis.com:443 "GET /bigquery/v2/projects/swast-scratch/datasets/schema_examples/tables/scalar_fields/data HTTP/1.1" 200 None

Closes #8826.

[plamut]

LGTM

[plamut]

Passing the args as keyword arguments makes this more readable. 👍