Add support for $rand aggregation operator.

[christophstrobl]

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Thanks!

Problem

MongoDB 4.4.2 introduced a new aggregation operator $rand that we want to support via the Aggregation Framework.
Details about the operator can be found within the MongoDB Reference Documentation.

Solution

Introduce MathOperators to the org.springframework.data.mongodb.core.aggregation package and add a Rand type that implements AggregationExpression. You may want to have a look at ArithmeticOperators to get an impression what this could look like.
Update org.springframework.data.mongodb.core.spel.MethodReferenceNode to add SpEL support for the operator via a rand() method.

Make sure to provide tests that assert the expression can be used within an pipeline and produces the desired command.
Again you may want to have a look at existing examples (eg. ArithmeticOperatorsUnitTests & SpelExpressionTransformerUnitTests).

Steps to Fix

• Claim this issue with a comment below and ask any clarifying questions you need
• Set up a repository locally following the Contributing Guidelines
• Try to fix the issue following the steps above
• Commit your changes and create a pull request

[ahmedmq]

@christophstrobl - Can I work on this issue please?

[christophstrobl]

sure - please assign it to yourself so others know you're taking care of it.
Happy to help if questions arise. The part with the MethodReferenceNode may benefit from changes in #3741 adding emptyRef().

[ahmedmq]

@christophstrobl - I am not seeing the button to assign the issue to myself, probably an access issue. could you assign it to me please.

[ahmedmq]

@christophstrobl - Any reason why we do not want to have the new rand() aggregation as part of the existing ArithmeticOperators? I was thinking we would mostly use the rand() operator to perform some arithmetic computation as part of the aggregation pipeline.

[christophstrobl]

It's fine to have them there. In case we want to introduce MathOperators at some point we can still delegate.