Support new aggregation features

[alcaeus]

Q	A
New Feature	yes
RFC	no
BC Break	no

Summary

This issue serves as an epic to track various aggregation features that are not supported in ODM yet.

New aggregation pipeline stages

• $currentOp
• $listLocalSessions
• $listSessions
• $merge
• $planCacheStats
• $sortByCount

In addition to these stages, the new $replaceWith, $set, and $unset stages are aliases for $replaceRoot, $addFields, and $project, respectively. We should consider adding those to the builder as well.

New aggregation operators

• $arrayToObject
• $dateFromParts
• $dateFromString
• $dateToParts
• $ltrim
• $mergeObjects
• $objectToArray
• $regexFind
• $regexFindAll
• $regexMatch
• $round
• $rtrim
• $trim
• All trigonometry operators
• All type operators

[alcaeus]

For the $currentOp, $listLocalSessions, and $listSessions stages: we may consider not adding these, as there is no concept for db.aggregate in the aggregation builder: it always runs on a specific collection. I'm not sure how to best handle that via the current API. At the same time, these commands are not likely to be used in your average ODM application, so I don't think it will be particularly bad if we don't add them now.

[markitosgv]

Any updates @alcaeus about this? Thanks!

[malarzm]

@markitosgv no updates on our end but we'll appreciate help adding any and all missing features :)