[elastic_agent] Add additional output metrics to dashboards

- beat.stats.libbeat.output.events.failed
- beat.stats.libbeat.output.events.toomany
- beat.stats.libbeat.output.events.dropped
- beat.stats.libbeat.output.batches.split

All four of these are useful to see if your agent is having non-fatal
errors when using the output which can result in degraded performance.

packages/elastic_agent/changelog.yml

@@ -1,4 +1,9 @@
 # newer versions go on top
+- version: "1.19.0"
+  changes:
+    - description: Add output metrics
+      type: enhancement
+      link: https://github.com/elastic/integrations/pull/8834
 - version: "1.18.0"
   changes:
     - description: Add metrics dashboard for httpjson, http_endpoint, filestream and CEL, fix decimal numbers on certain counters, add field mappings for filebeat_input.id, and component fields to filebeat_input logs

packages/elastic_agent/data_stream/elastic_agent_metrics/fields/beat-fields.yml

@@ -88,3 +88,15 @@
                   description: >
                     Number of write errors
 
+            - name: batches
+              type: group
+              description: >
+                Batches counters
+
+              fields:
+                - name: split
+                  type: long
+                  metric_type: counter
+                  description: >
+                    Number of batches split because they were too large
+