[Filebeat] Add separate error metrics for errors on the input and output sides of file ingestion

[rdner]

Describe the enhancement:

Currently we only have a total error counter processing_errors_total which accounts for both errors while reading from the file:

beats/filebeat/input/filestream/input.go

Line 678 in ca2d262

metrics.ProcessingErrors.Inc()

and while sending events to the output:

beats/filebeat/input/filestream/input.go

Line 728 in ca2d262

metrics.ProcessingErrors.Inc()

Users would like to see the separation of processing errors by "happened on the input side" and "happened on the output side":

• processing_errors_input
• processing_errors_output

Definition of Done

• Filebeat produces processing_errors_input and processing_errors_output as described above
• The dashboard in the Elastic Agent integration has the added metrics
• The documentation is updated accordingly

Describe a specific use case for the enhancement or feature:

The processing_errors_total metrics is not helpful for troubleshooting file ingestion errors, we need more granularity.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)