[Java][FlightRPC] Flight SQL JDBC driver does not expose per-batch app_metadata from FlightStream

从 ArrowFlightJdbcFlightStreamResultSet 及其现有的 getAppMetadata() 方法开始，然后跟踪 currentEndpointData 和 FlightStream.getLatestMetadata()。完成标准是 JDBC 调用方可以使用 unwrap() 获取最新的每批元数据，包括最后一个批次的尾随元数据，而无需访问原始 FlightClient。

Summary

The Flight SQL JDBC driver (flight-sql-jdbc-driver) currently exposes FlightInfo.app_metadata via ArrowFlightJdbcFlightStreamResultSet.getAppMetadata() (added in ~15.0), but per-batch app_metadata from FlightStream.getLatestMetadata() is received internally and silently discarded — it is never surfaced to JDBC callers.

Motivation

Servers use per-batch app_metadata on FlightData messages to communicate execution statistics (rows scanned, elapsed time, query state, etc.) — similar to what Trino exposes via QueryStats. This is particularly valuable for the final batch, which servers commonly use as a "trailing metadata" pattern to report post-execution stats.
Currently there is no way to access this information through the JDBC layer without dropping down to the raw FlightClient, which defeats the purpose of the JDBC driver abstraction.

Current behavior

FlightInfo.app_metadata → accessible via rs.unwrap(ArrowFlightJdbcFlightStreamResultSet.class).getAppMetadata() ✅
Per-batch FlightData.app_metadata → received by FlightStream.getLatestMetadata() internally, but never stored or exposed ❌

Also worth noting: getAppMetadata() returns a snapshot from query submission time only. For long-running queries using PollFlightInfo, updated app_metadata from subsequent poll cycles is never surfaced. This limits its usefulness for engines that progressively update stats during execution.

Requested change

Add a method to ArrowFlightJdbcFlightStreamResultSet (accessible via unwrap()) such as:

public ArrowBuf getLatestBatchMetadata() {
    if (currentEndpointData == null || currentEndpointData.getStream() == null) {
        return null;
    }
    return currentEndpointData.getStream().getLatestMetadata();
}

This would follow the same unwrap() pattern already established for getAppMetadata() and give callers access to per-batch metadata, including trailing stats on the final batch.

Component(s)

FlightRPC, Java