Add logging trace at debug level for the pipeline client. (elastic#9016)

* Add logging trace at debug level for the pipeline client.

To allow easier debugging when events are not sent by the output we
have added a few log statements at debug level for the onFilteredOut and
the onDroppedOnPublish events.

CHANGELOG.asciidoc

@@ -39,6 +39,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha1...master[Check the HEAD d
 - Log events at the debug level when dropped by encoding problems. {pull}9251[9251]
 - Refresh host metadata in add_host_metadata. {pull}9359[9359]
 - When collecting swap metrics for beats telemetry or system metricbeat module handle cases of free swap being bigger than total swap by assuming no swap is being used. {issue}6271[6271] {pull}9383[9383]
+- Adding logging traces at debug level when the pipeline client receives the following events: onFilteredOut, onDroppedOnPublish. {pull}9016[9016]
 
 *Auditbeat*
 

libbeat/publisher/pipeline/client.go

@@ -190,13 +190,15 @@ func (c *client) onPublished() {
 }
 
 func (c *client) onFilteredOut(e beat.Event) {
+	c.pipeline.logger.Debug("Pipeline client receives callback 'onFilteredOut' for event: %+v", e)
 	c.pipeline.observer.filteredEvent()
 	if c.eventer != nil {
 		c.eventer.FilteredOut(e)
 	}
 }
 
 func (c *client) onDroppedOnPublish(e beat.Event) {
+	c.pipeline.logger.Debug("Pipeline client receives callback 'onDroppedOnPublish' for event: %+v", e)
 	c.pipeline.observer.failedPublishEvent()
 	if c.eventer != nil {
 		c.eventer.DroppedOnPublish(e)