In clusters with many (5000+) timeseries collections, upgrade to FCV 9.0 could take a relatively long time, since each collection needs to be transformed to viewless format (similarly for downgrade).

It would be useful to warn users before the upgrade. Some possibilities are:

• Print a Server startup warning
• Print the warning message if setFCV is run without "confirm: true"
• Print the warning message if setFCV is run in "dryRun: true" mode
• Add a warning to the user facing documentation

Note: We should also consider generalizing this to other potentially long upgrade/downgrade processes, e.g. Authoritative Shards (SERVER-127975).