ethdb: more accurate batch size calculation (#23790)

This PR also counts the size of the key when calculating the size of a db batch

ethdb/leveldb/leveldb.go

@@ -455,7 +455,7 @@ type batch struct {
 // Put inserts the given value into the batch for later committing.
 func (b *batch) Put(key, value []byte) error {
 	b.b.Put(key, value)
-	b.size += len(value)
+	b.size += len(key) + len(value)
 	return nil
 }