Update resource-health-faq.yml

Added question related to common issue where availability metric is indicating down time even though there was no impact to the VM.

Author: kageor

articles/service-health/resource-health-faq.yml

@@ -113,6 +113,11 @@ sections:
         answer: |
           Based on the health status, Resource Health provides you with recommendations with the goal of reducing the time you spent troubleshooting. For available resources, the recommendations focus on how to solve the most common problems customers encounter. If the resource is unavailable due to an Azure unplanned event, the focus will be on assisting you during and after the recovery process. 
 
+      - question: |
+          Why am I seeing the VM availability metric reflect VM downtime when I observe no impact?
+        answer: |
+          The VM availability metric is calculated as an aggregation of the health checks stated [here](resource-health-checks-resource-types.md). When the guest agent is incorrectly installed or missing altogether, we interpret the guest heartbeat signal as unhealthy, and therefore emit a metric value of 0, which may be inaccurate. We have plans to move away from the aggregation model and rely only on one, unified high confidence signal from the host server. This work is already underway and will ensure our monitoring offerings give consistent values. We anticipate this work to be completed through the course of 2023.
+
 additionalContent: |
 
   ## Next steps