Create inputs metrics supplier

[xd4rker]

Closes Graylog2/graylog-plugin-enterprise#12201

Description

Introduced an InputsMetricsSupplier that collects the count of each input type. These counts will be submitted to PostHog during the scheduled execution of TelemetrySubmissionPeriodical as a single event named Inputs Metrics.

Example event properties:

{
  "cluster_id": "<UUID>",
  "beats_2_input": 2,
  "beats_kafka_input": 1,
  "gelf_http_input": 1,
  ...
}

How Has This Been Tested?

Tested on a local instance + unit tests.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have requested a documentation update.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[fpetersen-gl]

Thanks for the extensive documentation and tests! ❤️