AFAIK, there is no way to log the polling events from Apigee X in Cloud Logging. I don't believe Apigee X is instrumented to log a message with every health monitoring ping. It seems a reasonable feature request, that Apigee ought to provide some visibility into the status of the health checks. Like a log message that says "reporting healthy" or "reporting unhealthy" every once in a while. Maybe every 30 seconds. Probably not every ping. @madhurranjan maybe you have a view. or @gsjurseth ?
I think the best way, currently, to get logging, is to log from the "other side" - the endpoint that is being monitored.