Depends on:

• [Elastic Agent] Prototype a shipper gRPC input for Filebeat #35135
• Update the shipper monitoring implementation to be a variant of Beat monitoring elastic-agent#2580

Add input metrics for the shipper Filebeat input. The final definition of the useful metrics is to be defined, but could include:

• A gauge of the currently active RPC calls.
• Counters for the number of events and bytes received through the shipper
• A counter for the number of Publish calls that have failed or need to be retried.
• A gauge for the number of PersistedIndex subscriptions that are currently active.
• A counter for the number of PersistedIndex acknowledgements that were sent or have failed.