Does the ADC (Application Default Credentials) workflow only support Google Cloud APIs (for example, supports for Google Cloud Storage API, but not the Google Sheet API)?
Credentials are optional for the build constructor. Omit them and the Cloud Function service account will be used to authenticate against the Sheet.
from apiclient import discovery
sheets = discovery.build('sheets', 'v4')
SPREADSHEETID = '....'
result = sheets.spreadsheets().values().get(spreadsheetId=SPREADSHEETID, range='Sheet1!A:B').execute()
print result.get('values', [])