## What changes were proposed in this pull request?
This patch introduces an internal interface for tracking metrics and/or statistics on data on the fly, as it is being written to disk during a `FileFormatWriter` job and partially reimplements SPARK-20703 in terms of it.
The interface basically consists of 3 traits:
- `WriteTaskStats`: just a tag for classes that represent statistics collected during a `WriteTask`
The only constraint it adds is that the class should be `Serializable`, as instances of it will be collected on the driver from all executors at the end of the `WriteJob`.
- `WriteTaskStatsTracker`: a trait for classes that can actually compute statistics based on tuples that are processed by a given `WriteTask` and eventually produce a `WriteTaskStats` instance.
- `WriteJobStatsTracker`: a trait for classes that act as containers of `Serializable` state that's necessary for instantiating `WriteTaskStatsTracker` on executors and finally process the resulting collection of `WriteTaskStats`, once they're gathered back on the driver.
Potential future use of this interface is e.g. CBO stats maintenance during `INSERT INTO table ... ` operations.
## How was this patch tested?
Existing tests for SPARK-20703 exercise the new code: `hive/SQLMetricsSuite`, `sql/JavaDataFrameReaderWriterSuite`, etc.
Author: Adrian Ionescu <adrian@databricks.com>
Closes#18884 from adrian-ionescu/write-stats-tracker-api.
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