Document how auto-host generation can increase custom metric counts

[patsplat]

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• Fill out the template below. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• The pull request must only fix one issue, or add one feature, at the time.
• The pull request must update the test suite to demonstrate the changed functionality.
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What does this PR do?

This PR documents behavior of the dogapi library that can unexpectedly inflate billing.

Description of the Change

Adds documentation of how the host field is automatically set, and how that behavior can cause the creation of an excessive number of custom metrics.

Alternate Designs

One could also change the behavior of dogapi so that host detection was an opt-in rather than an auto-behavior. However that might break other customers installations.

Possible Drawbacks

The documentation may be less readable. This is worth a review to make sure that it appropriately matches the voice and interests of datadog.

Verification Process

It's a doc update

Additional Notes

This issue can potentially increase usage and customer costs, leading to a lack of trust in the library.

Release Notes

N/A

Review checklist (to be filled by reviewers)

• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
• PR should not have do-not-merge/ label attached.
• If Applicable, issue must have kind/ and severity/ labels attached at least.

[github-actions]

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