Amazon CloudWatch: How to find ARN of CloudWatch Log group

Go to Cloudwatch logs, find your log group, open it and you'll see a list of log streams. There is settings icon on top right:

Click it and you'll see an option to show stream arn:

Save the settings and you'll see stream arns. The part before semicolon looks like Log Group arn

The CloudWatch Group ARN format is arn:aws:logs:{region}:{account-id}:log-group:API-Gateway-Execution-Logs_{rest-api-id}/{stage-name}, cf. https://docs.aws.amazon.com/apigateway/latest/developerguide/set-up-logging.html