Add duo agent platform enabled metric

What does this MR do and why?

In order to track how many instances have DAP enabled, we add a new Service Ping metric.

References

Resolves #587250 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. In rails console regenerate service ping report

   report = Gitlab::Usage::ServicePingReport.for(output: :all_metrics_values)

2. Check the new metric report["settings"]["duo_agent_platform_enabled". It should exist.

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