Sharding time-series collections on the time field adds a lot of complexity for the sharding team and has questionable customer benefit. The meta field is providing enough cardinality to use as the sole shard key option for time-series collections.

The complexity is coming from that chunk boundaries do not overlap with bucket boundaries and buckets may contain logical data outside its chunk boundaries.

To be able to deprecate this feature we need warnings for users:

• startup warning when a time-series collection is sharded on time
• warning if the shardCollection command is performed on a time-series collection with time as shard key.

We might want to outright prohibit shardCollection with time as shard key rather than issue a warning.