Collect cache data over time

The ability to see cache hits on a request is great, but need two things to be truly useful:

• a) Group not by cache_bin, but by cache tags (top level tags, e.g. node:1 => node)
• b) Data needs to be collected over time

Over time collection is more like:

90% Hits, 10% Misses => Invalidation quality, Cache quality

See e.g. apc.php for an example of a good cache representation.

Another example would be https://github.com/FGM/heisencache that do something similar.