Add units to metric descriptions

[adjshort]

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What does this PR do?

Add units to metric descriptions in README.md

Description of the Change

The readme describes some metrics measured in seconds, some in milliseconds, and some were unlabeled. Updated the unlabeled ones after checking what's being sent in org.datadog.jenkins.plugins.datadog.listeners.

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• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
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