I'm going to build a web app and use BigQuery as a part of backend database, and I want to show the query cost information (ex. 1.8 sec elapsed, 264.9 MB processed) in the app.
I know we can check the BigQuery's query information inside GCP, but how do we I get that information from BigQuery API?
The information you are interested in is present in the job statistics.
See jobs.get for more details: https://cloud.google.com/bigquery/docs/reference/rest/v2/jobs/get
The dry-run sample may be of interest as well, though you can get the stats from a real invocation as well (dry run is for estimating costs without executing the query):
https://cloud.google.com/bigquery/docs/samples/bigquery-query-dry-run