Docs: Add GCM update to what's new (#35211)

* content updates

* Update docs/sources/whatsnew/whats-new-in-v8-0.md

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

* Update docs/sources/whatsnew/whats-new-in-v8-0.md

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

docs/sources/datasources/google-cloud-monitoring/_index.md

@@ -111,7 +111,7 @@ When the operator is set to `=~` or `!=~` it is possible to add regular expressi
 
 Preprocessing options are displayed in the UI when the selected metric has a metric kind of `delta` or `cumulative`. If the selected metric has a metric kind of `gauge`, no pre-processing option will be displayed.
 
-If you select 'Rate', data points are aligned and converted to a rate per time series. If you select 'Delta', data points are aligned by their delta (difference) per time series.
+If you select `Rate`, data points are aligned and converted to a rate per time series. If you select `Delta`, data points are aligned by their delta (difference) per time series.
 
 #### Grouping
 

docs/sources/whatsnew/whats-new-in-v8-0.md

@@ -189,6 +189,15 @@ You can now set a custom line limit for logs queries instead of accepting the pr
 
 Response values were always interpreted as strings in Elasticsearch responses, which caused issues with some visualization types that applied logic based on numeric values. We now apply some heuristics to detect value types from the first non-empty value in each response.
 
+#### Google Cloud Monitoring data source
+
+In a prior release, Cloud Monitoring added _preprocessing_ support in their query editor. This capability has been added to the Cloud Monitoring data source in Grafana.
+
+Whenever a metric is selected in the query editor, a suitable preprocessing option is automatically selected for you. To avoid breaking changes, preprocessing is not enabled by default on existing queries. If you want to use preprocessing for existing queries, you have to manually select one in the query editor.
+
+[Google Cloud Monitoring data source]({{< relref "../datasources/google-cloud-monitoring/_index.md#pre-processing" >}}) was updated as a result of this change.
+
+
 #### Graphite data source
 
 [Graphite data source]({{< relref "../datasources/graphite.md" >}}) was updated as a result of these changes.