[ReleasePR monitor] Expose autoMitigate flag (#1589)

Create to sync Azure/azure-rest-api-specs#13109
[ReCreate this PR](https://github.com/azure-resource-manager-schemas/compare/master...AzureSDKAutomation:sdkAuto/monitor?expand=1)

Author: msftbot[bot]

schemas/2018-04-16/Microsoft.Insights.json

@@ -312,6 +312,18 @@
           ],
           "description": "Action descriptor."
         },
+        "autoMitigate": {
+          "oneOf": [
+            {
+              "type": "boolean",
+              "default": false
+            },
+            {
+              "$ref": "https://schema.management.azure.com/schemas/common/definitions.json#/definitions/expression"
+            }
+          ],
+          "description": "The flag that indicates whether the alert should be automatically resolved or not. The default is true."
+        },
         "description": {
           "type": "string",
           "description": "The description of the Log Search rule."