change mongodb atlas sysmem metrics to have Kb units

[tdelacour]

What does this PR do?

This updates the mongodb integration metadata on mongodb.atlas.system.mem metrics to have units=kilobytes.

Motivation

Mongodb Atlas publishes these metrics in kilobytes rather than bytes. Updating the metadata on the datadog side will correct the displayed data in datadog for customers and save them the effort of manually reconfiguring this.

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• PR has a meaningful title or PR has the no-changelog label attached
• Feature or bugfix has tests
• Git history is clean
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