This PR:
(1) list the metrics values needed
(2) add the _metric_header_for_usage method to the base credential class, which is used by the before_request method to add the metrics header for token usage. Children credentials classes can override this method for token usage metrics.
This PR: (1) list the metrics values needed (2) add the
_metric_header_for_usagemethod to the base credential class, which is used by thebefore_requestmethod to add the metrics header for token usage. Children credentials classes can override this method for token usage metrics.internal doc: go/googleapis-auth-metric-design