Clarify classification of metrics submitted via DogStatsD or Agent

[kaveet]

What does this PR do?

Adds guidance for the treatment of metrics as Custom Metrics when using DogStatsD or Agent Checks.

This language appears in /metrics/custom_metrics/ but is separately useful in the context of billing.

Motivation

I encountered conflicting information when researching this topic--this language from the Custom Metrics page helped me clarify categorization of Custom Metrics. For example, the Node.js StatsD client (npm/hot-shots) appears in the list of "more than 500 integrations" but ultimately produces Custom Metrics via DogStatsD.

Additional Notes

I avoided reorganizing the links in this document to avoid a large diff.

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Reviewer checklist

• [ ] Review the changed files.
• [ ] Review the URLs listed in the Preview section.
• [ ] Check images for PII
• [ ] Review any mentions of "Contact Datadog support" for internal support documentation.

[alai97]

Hi @kaveet, thanks for opening this PR to clarify the custom metric definition! I created an editorial review card (DOCS-4155).