Add expected APM server response logs to troubleshooting

Author: felixbarny

docs/troubleshooting.asciidoc

@@ -46,7 +46,19 @@ Each request should at least add some lines similar to these in the logs:
 ----
 
 If you don't see anything in your logs,
-the technology stack you are using is probably not <<supported-technologies,supported>>.
+the technology stack you are using is probably not <<supported-technologies-details,supported>>.
+
+After that, you should see logs indicating that the agent has successfully sent data to the APM server:
+
+----
+[apm-reporter] DEBUG co.elastic.apm.report.ApmServerHttpPayloadSender - Sending payload with 1 elements to APM server http://localhost:8200
+[apm-reporter] DEBUG co.elastic.apm.report.ApmServerHttpPayloadSender - APM server responded with status code 202
+----
+
+If the APM server responds with a 400,
+it could indicate JSON validation errors.
+In that case,
+please turn on `TRACE` logging and send us the logged JSON payload.
 
 [float]
 [[trouble-shooting-debugging]]