APCu: Undefined index: sum

On our servers we are experiencing a bug (especially after running for some days).

The APCu seems to get currupted (?), resulting in a 

`Undefined index: sum` on `getMetricFamilySamples()`

I was able to reproduce this by removing just the ":sum:value"-key.

Some additional checks would be great. Currently, I am mitigating by catching the ErrorException and calling `apcu_clear_cache`.