Update metric mapping

Author: snehithm

articles/azure-monitor/platform/alerts-understand-migration.md

@@ -27,6 +27,8 @@ Although the tool can migrate almost all classic alert rules, there are some exc
 
 If your subscription has any such classic rules, you must migrate them manually. Because we can't provide an automatic migration, any existing, classic metric alerts of these types will continue to work until June 2020. This extension gives you time to move over to new alerts. However, no new classic alerts can be created after August 2019.
 
+Besides the above listed exceptions, if your classic alert rules are invalid i.e. they are on deprecated metrics ([list below](deprecated-metrics)) or resources that have been deleted, they will not be migrated during voluntary migration. Any such invalid classic alert rules will be deleted when automatic migration happens. 
+
 ### Guest metrics on virtual machines
 
 Before you can create new metric alerts on guest metrics, the guest metrics must be sent to the Azure Monitor custom metrics store. Follow these instructions to enable the Azure Monitor sink in diagnostic settings:
@@ -55,6 +57,20 @@ Classic alert rules on Percent metrics must be migrated based on [the mapping be
 
 Classic alert rules on AnonymousThrottlingError, SASThrottlingError and ThrottlingError must be split into two new alerts because there is no combined metric that provides the same functionality. Thresholds will need to be adapted appropriately.
 
+## Classic alert rules on deprecated metrics
+
+These are classic alert rules on metrics which were supported for a while before they were eventually deprecated. A small percentage of customer might have invalid classic alert rules on such metrics. Since these alert rules are invalid, they won't be migrated. 
+
+|-------------|----------------- |
+| Resource type| Deprecated metric |
+| Microsoft.DBforMySQL/servers | compute_consumption_percent, compute_limit |
+| Microsoft.DBforPostgreSQL/servers | compute_consumption_percent, compute_limit |
+| Microsoft.Network/publicIPAddresses | defaultddostriggerrate |
+| Microsoft.SQL/servers/databases | service_level_objective;storage_limit;storage_used;throttling;dtu_consumption_percent;storage_used |
+| Microsoft.Web/hostingEnvironments/multirolepools | averagememoryworkingset |
+| Microsoft.Web/hostingEnvironments/workerpools | bytesreceived; httpqueuelength |
+
+
 ## Rollout phases
 
 The migration tool is rolling out in phases to customers that use classic alert rules. Subscription owners will receive an email when the subscription is ready to be migrated by using the tool.