MINOR: cache: add a counter for cache hits serving early hints

MINOR: cache: add a counter for cache hits serving early hints

Add a cache_hint_hits counter to the per-proxy HTTP stats, incremented
each time the cache emits a 103 Early Hints response. It is reported
in the CSV/typed outputs, on the stats page next to the cache hits,
and exported to prometheus as http_cache_hint_hits_total. Since the
new field changes the shm stats file mapping, its minor version is
bumped along with the object size assertion.