Note that cache impls are monotonically increasing counters in Javadoc

[Amemeda]

Update SPI javadoc to note that cache impls are monotonically increasing counters.

Added to javadoc in BlockCache.Stats.java interface.

This pr closes #4530

[Amemeda]

This was the only area I could find "cache impls" using the counters, if there are more, I may need to pointed in the right direction

[DomGarguilo]

I think BlockCacheMetrics is the wrong spot for this comment since its internal code that nobody implements. The intent should be to notify plugin authors. I think the BlockCache.Stats interface is the correct spot for things.

This BlockCache implementation is a monotonically increasing counter, the counter should not reset or
decrease the counts as the metrics rely on these values being monotonically increasing.

I also think this could be improved in a few ways

1. the counts are monotonic not the cache. Something like "All counts returned by this interface must be monotonically increasing" seems more correct to me
2. since the goal is to outline the contract we should say "must never" instead of "should not" so it reads as a requirement instead of a suggestion
3. I think the why could be improved. "shouldn't reset because the metrics rely on it not resetting" is kinda what its saying now which is circular reasoning. Something that explains that the values are read periodically and what matters is how much things go up between those readings so an implementation must never reset or decrease them.