[VL] Iceberg metric NumDeletes

### Description

Iceberg metric numSplits and numDeletes gets from ParquetReader and update the metric when creating DatasourceRDD, the logic is different in Gluten.
Create new metric numSplit to report the splits number.
But numDeletes should get from Velox metric which does not report now.

### Gluten version

None

Start at DatasourceRDD creation and compare how ParquetReader supplies numSplits and numDeletes. Then inspect the Velox metric reporting path for delete counts; the work is done when the split count and delete count are both reported consistently for Iceberg data sources.

Written by the indexing model from the issue text.