Add support for $round aggregation expression [DATAMONGO-2370]

[spring-projects-issues]

Christoph Strobl opened DATAMONGO-2370 and commented

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Reference URL: https://docs.mongodb.com/manual/reference/operator/aggregation/round/

Referenced from: pull request #803, and commits cd4e406, f963bbf, 9f4872d, 5cfc5f3, 2dde2c7, 7abf69e