Adding Cache::flexible to the metrics

[mostafa-rz]

Hi,
The metrics are the first page visited when someone logs in. Currently, I can cache them, but the underlying function is Cache::remember. If the cache key isn't set, the page load time is affected. Can you please add the feature to use Cache::flexible? I saw the source code, and introducing staleFor() method, same as the cacheFor(), and reacting to it if it is set, can be easily done. It is also backward compatible.