Added custom metrics to Widget Builder (#10195)

* Added custom metrics

* Update docs/product/dashboards/widget-builder/index.mdx

* Added screenshot custom metrics

Added screenshot for custom metrics widget builder

* Update docs/product/dashboards/widget-builder/index.mdx

* Update docs/product/dashboards/widget-builder/index.mdx

Co-authored-by: vivianyentran <20403606+vivianyentran@users.noreply.github.com>

* Update docs/product/dashboards/widget-builder/index.mdx

Co-authored-by: vivianyentran <20403606+vivianyentran@users.noreply.github.com>

---------

Co-authored-by: vivianyentran <20403606+vivianyentran@users.noreply.github.com>

Author: ale-cota

docs/product/dashboards/widget-builder/img/widget-builder-custom-metrics.png

@@ -74,6 +74,20 @@ Choosing "Releases (sessions, crash rates)" allows you to query properties speci
 - Crash rates for your latest release
 - Session count across releases
 
+### Custom Metrics
+
+Choosing "Custom Metrics" allows you to query your custom metrics data in the same way you would in the [Metrics Explorer Page](/product/metrics/metrics-explorer-page/). Some example widgets might include:
+
+- Number of user signups
+- API request payloads
+- Processing time for a task
+
+To learn more about the types of metrics you can send to Sentry and what you can do with them, see [Metrics](/product/metrics). 
+
+In order to optimize the experience for querying metrics, the widget builder UI is slightly different than it is for other datasets. You can query metrics the same way you would in the [Metrics Explorer Page](/product/metrics/metrics-explorer-page/). With metrics, you can choose how they're aggregated and grouped, as well as define equations with multiple metrics. 
+
+![Custom metrics widget builder.](./img/widget-builder-custom-metrics.png)
+
 ## Choose Your Columns
 
 The option to set columns is only available for [table visualizations](/product/dashboards/widget-builder/#table).