Run DB Checks Fails with 504 Gateway timeout with PMM-Server monitoring high number of DB Instances

User Impact: For a User using PMM for monitoring a high number of DB instances, it might not really be possible to run the DB checks on all monitored Instances using Run DB Checks Button. 

During the Performance Testing Instance Setup for PMM 2.23.0 RC We tried to run DB Checks on a PMM-Server instance monitoring roughly 380 Services

The user sees a totally unfriendly error on the UI "Unknown Error" and the request times out. 

At the end of the request we show them, DB checks were executed Successfully which is more confusing. 

Expected Behaviour: Request timeout error should be avoided, maybe we could break the requests into smaller. 

At the moment, we don't have a way to give users a better view to watch progress for these checks running on monitored DB services.