Pick [Fix](group commit) Fix group commit block queue mem estimate faule
#35314
## Proposed changes
Issue Number: close #xxx
<!--Describe your changes.-->
**Problem:** When `group commit=async_mode` and NULL data is imported
into a `variant` type column, it causes incorrect memory statistics for
group commit backpressure, leading to a stuck issue. **Cause:** In group
commit mode, blocks are first added to a queue in batches using `add
block`, and then blocks are retrieved from the queue using `get block`.
To track memory usage during backpressure, we add the block size to the
memory statistics during `add block` and subtract the block size from
the memory statistics during `get block`. However, for `variant` types,
during the `add block` write to WAL, serialization occurs, which can
merge types (e.g., merging `int` and `bigint` into `bigint`), thereby
changing the block size. This results in a discrepancy between the block
size during `get block` and `add block`, causing memory statistics to
overflow.
**Solution:** Record the block size at the time of `add block` and use
this recorded size during `get block` instead of the actual block size.
This ensures consistency in the memory addition and subtraction.
## Further comments
If this is a relatively large or complex change, kick off the discussion
at [dev@doris.apache.org](mailto:dev@doris.apache.org) by explaining why
you chose the solution you did and what alternatives you considered,
etc...
## Proposed changes
Issue Number: close #xxx
<!--Describe your changes.-->
