documentation for ddinjector metrics

[alexn-dd]

What does this PR do? What is the motivation?

This adds public descriptions to metrics reported by ddinjector. These metrics are meant for internal development teams only.
https://datadoghq.atlassian.net/browse/DOCS-13479
https://datadoghq.atlassian.net/browse/ASUP-63

Merge instructions

Merge readiness:

• Ready for merge

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Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/alexn-dd/ddinjector_metrics/data_security/agent