metrics.py: punctuation fix

[yzgyyang]

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What does this PR do?

This fixes the generated documentation style as shown in https://datadogpy.readthedocs.io/en/latest/#datadog.api.Metric.send.

Description of the Change

Alternate Designs

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Additional Notes

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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.
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[github-actions[bot]]

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