Ingest sampling support

[shawncao]

Some use case needs sampling support during data ingestion for some super heavy data source, users can get insights without scan full data.

a few initial thoughts

• sampling policy: random sampling, shard key sampling, partition level sampling
• sampling ratio: percentage

[chenqin]

there are more into sampling when it comes to data science and statistical (old name of ML) use cases. we can start from simple one pass algorithm.