Report per input metrics for dropped events in the output

[AndersonQ]

Describe the enhancement:

The input metrics should contain data about the number of events dropped by each input. This would help narrow down the source of problems to a particular input (and hence a particular integration data stream in the case of Elastic Agent).

Describe a specific use case for the enhancement or feature:

• Diagnostic dumps from Elastic Agent will contain dropped metrics that are specific to an integration data stream (via input_metrics.json).
• Input metric dashboards in the Elastic Agent fleet integration can display dropped/filtered charts, providing a visual representation of the issue.
• Standalone beat users can access detailed information about which input is causing drops through the HTTP monitoring API.

[botelastic]

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