perf: Avoid copying metadata for each data file in summary (#2674)

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Resolves #2673

# Rationale for this change

`_SnapshotProducer._summary()` copies the metadata for _every_ added /
deleted DataFile. This is pretty expensive. Instead we just copy it once
at the beginning of the function and use the same value each DataFile.

On my data, which overwrites a few million rows at a time, I saw the
time for `table.overwrite` go from ~20 seconds to ~6 seconds.

## Are these changes tested?

Yes, existing unit / integration tests

## Are there any user-facing changes?

Just faster writes :)

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Author: Anton-Tarazi

pyiceberg/table/update/snapshot.py

@@ -240,8 +240,11 @@ def _write_delete_manifest() -> List[ManifestFile]:
     def _summary(self, snapshot_properties: Dict[str, str] = EMPTY_DICT) -> Summary:
         from pyiceberg.table import TableProperties
 
+        # avoid copying metadata for each data file
+        table_metadata = self._transaction.table_metadata
+
         partition_summary_limit = int(
-            self._transaction.table_metadata.properties.get(
+            table_metadata.properties.get(
                 TableProperties.WRITE_PARTITION_SUMMARY_LIMIT, TableProperties.WRITE_PARTITION_SUMMARY_LIMIT_DEFAULT
             )
         )
@@ -250,23 +253,21 @@ def _summary(self, snapshot_properties: Dict[str, str] = EMPTY_DICT) -> Summary:
         for data_file in self._added_data_files:
             ssc.add_file(
                 data_file=data_file,
-                partition_spec=self._transaction.table_metadata.spec(),
-                schema=self._transaction.table_metadata.schema(),
+                partition_spec=table_metadata.spec(),
+                schema=table_metadata.schema(),
             )
 
         if len(self._deleted_data_files) > 0:
-            specs = self._transaction.table_metadata.specs()
+            specs = table_metadata.specs()
             for data_file in self._deleted_data_files:
                 ssc.remove_file(
                     data_file=data_file,
                     partition_spec=specs[data_file.spec_id],
-                    schema=self._transaction.table_metadata.schema(),
+                    schema=table_metadata.schema(),
                 )
 
         previous_snapshot = (
-            self._transaction.table_metadata.snapshot_by_id(self._parent_snapshot_id)
-            if self._parent_snapshot_id is not None
-            else None
+            table_metadata.snapshot_by_id(self._parent_snapshot_id) if self._parent_snapshot_id is not None else None
         )
 
         return update_snapshot_summaries(