[Elastic Agent] Implement input metrics for the shipper Filebeat input

[cmacknz]

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.

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[botelastic]

Hi!
We just realized that we haven't looked into this issue in a while. We're sorry!

We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1.
Thank you for your contribution!