Previewing a metric lets you view a metric via a live query.

You can preview a metric during metric creation to verify its behavior is what you expect.
You can preview a metric after the metric has already been running. In this case, the chart you see during preview may not be the same as what you see in Explorer or in the incident analysis pane. This is because the metric data in Explorer and incident analysis is collected and stored on a periodic basis. The data may have changed since it was collected. The preview is showing a live query, not the data that was collected for the metric.

Preview a metric

Create a metric or edit an existing metric.
Go to Step 3 (Preview), set the time range for your preview, and click Preview.