[APM] Document _inspect flag (#117635)

x-pack/plugins/apm/dev_docs/query_debugging_in_development_and_production.md

@@ -0,0 +1,17 @@
+# Query Debugging
+
+When debugging an issue with the APM UI it can be very helpful to see the exact Elasticsearch queries and responses that was made for a given API request. 
+To enable debugging of Elasticsearch queries in APM UI do the following:
+
+1. Go to "Stack Management" 
+2. Under "Kibana" on the left-hand side, select "Advanced Settings"
+3. Search for "Observability"
+4. Enable "Inspect ES queries" setting
+5. Click "Save"
+
+When you navigate back to APM UI you can now inspect Elasticsearch queries by opening your browser's Developer Tools and selecting an api request to APM's api. 
+There will be an `_inspect` key containing every Elasticsearch query made during that request including both requests and responses to and from Elasticsearch.
+
+![image](https://user-images.githubusercontent.com/209966/140500012-b075adf0-8401-40fd-99f8-85b68711de17.png)
+
+

x-pack/plugins/apm/readme.md

@@ -13,6 +13,9 @@
 ## Tooling
 - [VSCode setup instructions](./dev_docs/vscode_setup.md)
 - [Github PR commands](./dev_docs/github_commands.md)
+- [Synthtrace (data generation)](https://github.com/elastic/kibana/blob/main/packages/elastic-apm-synthtrace/README.md)
+- [Query debugging in development and production](./dev_docs/query_debugging_in_development_and_production.md)
 
 ## Other resources
 - [Official APM UI settings docs](https://www.elastic.co/guide/en/kibana/current/apm-settings-in-kibana.html)
+- [Reading Material](./dev_docs/learning_material.md)