Add instructions for locating the resource ID (Azure#23164)

Author: scottaddie

sdk/monitor/azure-monitor-query/README.md

@@ -283,6 +283,12 @@ MetricsQueryAsyncClient metricsQueryAsyncClient = new MetricsQueryClientBuilder(
 
 ### Get metrics
 
+A resource ID, as denoted by the `{resource-id}` placeholder in the sample below, is required to query metrics. To find the resource ID:
+
+1. Navigate to your resource's page in the Azure portal.
+2. From the **Overview** blade, select the **JSON View** link.
+3. In the resulting JSON, copy the value of the `id` property.
+
 <!-- embedme ./src/samples/java/com/azure/monitor/query/ReadmeSamples.java#L161-L188 -->
 ```java
 Response<MetricsQueryResult> metricsResponse = metricsQueryClient