fix azure sql db discovery metric (#1419)

Author: yduartep

csp-mixin/signals/azuresqldb.libsonnet

@@ -7,7 +7,7 @@ function(this)
     instanceLabels: this.instanceLabels,
     aggLevel: 'none',
     discoveryMetric: {
-      azuremonitor: 'azure_microsoft_storage_storageaccounts_blobservices_blobcount_average_count',
+      azuremonitor: 'azure_microsoft_sql_servers_databases_sessions_count_average_count',
       azuremonitor_agentless: self.azuremonitor,
     },