KVBucket::getValue can lead to incorrectly incrementing "failure_get" a failover trigger stat

From linked CBSE we see this warning many times.

WARNING (bucket) KVBucket::getValue failed get for item vb:843, it.seqno:-4, status:1

This code is driven by the expiry pager, it is trying to load the value of a now expired item which is tagged as "xattr":

void KVBucket::deleteExpiredItem(Item& it,

                                 time_t startTime,

                                 ExpireBy source) {

    VBucketPtr vb = getVBucket(it.getVBucketId());

    if (vb) {

        // MB-25931: Empty XATTR items need their value before we can call

        // pre_expiry. These occur because the value has been evicted.

        if (mcbp::datatype::is_xattr(it.getDataType()) && it.getNBytes() == 0) {

            getValue(it);

        }

However "seqno:-4" indicates this is really a state_non_existent_key StoredValue, which occurs due to bgfetch. Each time getValue fails to just find the document, because it genuinely doesn't exist we still increment failure_get

This is a side-effect of the bug described in MB-47952 - having deleted, non-resident documents in memory which (incorrectly) have the XATTR datatype set. As such we attempt to read a document which no longer exists, and hence the above error is seen (and failure stat incremented).