In Structured Streaming, we have the concept of Triggers. With a trigger like Trigger.Once(), the semantics are to process all the data available to the datasource in a single micro-batch. However, this semantic can be broken when data source options such as `maxOffsetsPerTrigger` (in the Kafka source) rate limit the amount of data read for that micro-batch.
We propose to add a new interface `SupportsAdmissionControl` and `ReadLimit`. A ReadLimit defines how much data should be read in the next micro-batch. `SupportsAdmissionControl` specifies that a source can rate limit its ingest into the system. The source can tell the system what the user specified as a read limit, and the system can enforce this limit within each micro-batch or impose it's own limit if the Trigger is Trigger.Once() for example.