### Description

![image](https://github.com/elastic/elasticsearch/assets/76245/d0443170-0419-4759-b505-7620ff3d6594)

The execution time of grouping stats by multiple keywords has increased: 1.2 to 1.6 seconds (with doc_partitioning), 4.3 to 6.7 seconds (with shard_partitioning). The equivalent query with the _search API takes 1.8 seconds.

`from nyc_taxis | ... | stats c = count(dropoff_datetime) by trip_type, payment_type, vendor_id`

This regression occurred since July 2, 2023, and I suspect it may be due to the PR: [Support multivalued fields on arbitrary grouping](https://github.com/elastic/elasticsearch-internal/pull/1340).

/cc @nik9000
x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/operator/BatchEncoder.java
*/
    protected final void endPosition() {
        valueOffsets = ArrayUtil.grow(valueOffsets, currentValue + 1);
        valueOffsets[currentValue] = bytes.length();
        positionCount++;
    }
    /**
     * Implementations of {@link #readNextBatch} should call this to encode
     * an entirely null position.
     */
    protected final void encodeNull() {
        startPosition();
        addingValue();
        endPosition();
    }
    /**
     * The number of bytes in all entries in the batch.
     */
    final int bytesLength() {
        return bytes.length();
    }
    /**
     * The maximum batch size. This starts the same as the constructor parameter
     * but will grow if a single entry doesn't fit into the batch.
     */
    final int bytesCapacity() {
        return bytes.bytes().length;
    }
    private static final VarHandle intHandle = MethodHandles.byteArrayViewVarHandle(int[].class, ByteOrder.nativeOrder());
    protected abstract static class Ints extends BatchEncoder {