fix(netapp-harvest-exporter): guard AbsentNetAppSnapmirrorLabelsMaia against replication-sink regions

[chuan137]

Problem

AbsentNetAppSnapmirrorLabelsMaia was firing as a false positive in regions that act as pure replication sinks (e.g. na-us-3).

In these regions all NetApp volumes scraped locally are volume_type=dp (replica destinations). The :enhanced recording rule joins on volume_type="rw" source volumes, which don't exist here, so netapp_snapmirror_labels:local is always empty and :maia is always absent — by design, not due to a real problem.

Verified across all production regions:

Region	:local count
eu-de-1	500
eu-de-2	1068
eu-nl-1	41
na-us-1	110
na-us-3	0 (replication sink — all dp volumes)
ap-jp-1	91
ap-au-1	122
ap-ae-1	138

Fix

Guard the alert with count(netapp_snapmirror_labels:local) > 0 — only fire when the region has local snapmirror source relationships to track.

Test plan

• Verify alert no longer fires in na-us-3 after deploy
• Verify alert still fires in a region where :maia goes absent (e.g. by checking alert history in eu-de-1)