private BsonValue lowerBoundOfBucket(BsonDocument bucket) {
        return bucketBounds(bucket).get(MIN_FIELD);
    }
The Sample Splitter.

<p>Uses the document size and 5% sampling rate of the collection to generate chunks for the
collection.

<pre>
If all of the following conditions are true, $sample uses a pseudo-random cursor to select the N documents:

     $sample is the first stage of the pipeline.
     N is less than 5% of the total documents in the collection.
     The collection contains more than 100 documents.
     If any of the previous conditions are false, $sample:

Reads all documents that are output from a preceding aggregation stage or a collection scan.
     Performs a random sort to select N documents.
</pre>
Copy from com.ververica:flink-connector-mongodb-cdc:2.3.0.
inlong-sort/sort-flink/sort-flink-v1.13/sort-connectors/mongodb-cdc/src/main/java/org/apache/inlong/sort/cdc/mongodb/source/assigners/splitters/SampleBucketSplitStrategy.java