As part of the time-series query rewrites, MatchExpression predicates like {meta.host: {$eq: "host_0"}} are mapped onto bucket-level fields and pushed in front of the _internalUnpackBucket stage so they can potentially utilize an index.

We should do something similar to aggregation comparison predicates like {$expr: {$eq: ["$meta.host", "host_0]}} which currently do not participate in this mapping optimization and thus require all the buckets to be unpacked.