ep_num_ops_get_meta may not count requests which are non-resident

(Spun out from MB-20328):

We have a bug in the ep_num_ops_get_meta stat which was introduced in 4.0.0:

• Prior to that release (e.g. 3.1.5), ep_num_ops_get_meta counted all requests - successful or unsuccessful. See http://src.couchbase.org/source/xref/3.1.5/ep-engine/src/ep.cc#1769 - note that if a StoredValue is found - v is non-null - then we increment stats.numOpsGetMeta. If v is null then we always call addTempItemForBgFetch - which essentially will always result suspending the request (EWOULDBLOCK) while the disk fetch occurs, after which it'll retry the request - i.e EventuallyPersistentStore::getMetaData will be called again, v will be non-NULL and stats.numOpsGetMeta will get incremented.
• In 4.0.0, when bloomfilters were introduced, ep_num_ops_get_meta no longer counts requests to items which are not resident and the bloomfilter tells us are not on disk - the else clause no longer unconditionally triggers a BGfetch - and crucially doesn't increment stats.numOpsGetMeta

    } else {

1994        // The key wasn't found. However, this may be because it was previously

1995        // deleted or evicted with the full eviction strategy.

1996        // So, add a temporary item corresponding to the key to the hash table

1997        // and schedule a background fetch for its metadata from the persistent

1998        // store. The item's state will be updated after the fetch completes.

1999        //

2000        // Schedule this bgFetch only if the key is predicted to be may-be

2001        // existent on disk by the bloomfilter.

2002

2003        if (vb->maybeKeyExistsInFilter(key)) {

2004            return addTempItemForBgFetch(lh, bucket_num, key, vb, cookie, true);

2005        } else {

2006            return ENGINE_KEY_ENOENT;

2007        }

We need to fix the code so ep_num_ops_get_meta is incremented for all results of the request - i.e. also when the above code directly returns ENGINE_KEY_ENOENT.