kv_ops{get} is not incremented for subdoc lookup operations

kv_ops{op="get"} is documented as "The number of successful get operations". On versions before 7.6.0, it is updated for GET and GAT only.

On versions 7.6.0 and later, it is updated for GET/GAT/SUBDOC_MULTI_LOOKUP.

SUBDOC_MULTI_LOOKUP (like many other operations) might require a BGFetch for a document.

On versions 7.0 and later, ns_server derives the cache miss ratio (visible in the UI) as 100 * ep_bg_fetched / kv_ops{op="get"}. ns_server will additionally alert the customer for high cache miss ratio.

SUBDOC_MULTI_LOOKUP not being counted in kv_ops (but triggering BGFetch) means that cache miss ratio can go off the scale on <7.6.0.

This is less likely to happen now in 7.6.0, however kv_ops does not account for all operations which can cause BGFetch, so it is still possible to observe high cache miss ratio due to other operations on 7.6.0.