Add azure_natgateway.yaml alert rules for Azure NAT Gateway metrics

[ian-flores]

Azure NAT Gateway metrics (DroppedPackets, SNATConnectionCount, etc.) are currently scraped via prometheus.exporter.azure in Grafana Alloy when public_subnet_cidr is configured, but there are no corresponding Grafana alert rules for these metrics.

The existing natgateway.yaml covers AWS CloudWatch only. Azure NAT Gateway metrics have no alert coverage yet.

Metrics currently scraped:

• PacketCount
• ByteCount
• DroppedPackets
• TotalConnectionCount
• SNATConnectionCount

Acceptance criteria:

• Create azure_natgateway.yaml with alert rules for at minimum DroppedPackets and SNATConnectionCount
• Add the new file to the _create_alert_configmap list in grafana_alloy.py
• Add test coverage in test_azure_alert_files.py