Add "X-cache: PASS" support in cacheHits module

Hi,

I don't know if this PR could have some interest for you, but in case ...

This PR add X-cache PASS metrics.

In our Varnish, we have HIT, MISS and PASS.

PASS tell us there is a rule which exclude cache on this request, and tell us which rules.

And PASS request, is a thing that we have to look, because it means that every hit go to the backend !

;) Thanks