Improve/standardize rate limiting logic in Monitor

[DomGarguilo]

This PR uses Guavas Suppliers.memoizeWithExpiration() for caching, replacing the manual caching logic. This allows us to:

• Remove synchronized from the get methods since Suppliers.memoizeWithExpiration() is thread safe
• Remove the class-level maps that were previously being used for caching. Now, the fetch() methods that update the data, directly return the data instead of using these maps. These fetch() methods are called directly from the suppliers now.
• Remove the age-off logic that was being done in the fetch() methods. It should be noted that the functionality has slightly changed. Since we just re-create a new map whenever the data is refreshed in the fetch() methods, there are no old values that need to be aged off.

[keith-turner]

Remove the age-off logic that was being done in the fetch() methods. It should be noted that the functionality has slightly changed. Since we just re-create a new map whenever the data is refreshed in the fetch() methods, there are no old values that need to be aged off.

That is a nice simplification.

[DomGarguilo]

Remove the age-off logic that was being done in the fetch() methods. It should be noted that the functionality has slightly changed. Since we just re-create a new map whenever the data is refreshed in the fetch() methods, there are no old values that need to be aged off.

That is a nice simplification.

Yea definitely nice to be able to remove some code. Wasn't sure if this would be an issue though since before, the maps would contain values that would only be aged off after they were 15 minutes old. And now, since the data is recreated once every minute, all of the returned data will only ever be that old. Not too sure what the implications of this are or what the old values were being used for.

[keith-turner]

Wasn't sure if this would be an issue though since before, the maps would contain values that would only be aged off after they were 15 minutes old.

I misread that code when I was reviewing. I thought they were aging off after 15 milliseconds, so did not think it was an issue. But its actually 15 minutes. It would be good to understand that change in behavior a bit better. So it seems like some things used to hang around for 15 minutes in the monitor display even after they were gone?