Introduce new getter for prepared result col specs

Current getter (`get_result_set_col_specs`) always returns columns based
on initial result metadata (which can be stale), and this unfortunately
can't be changed due to backwards compatibility.
New getter is introduced: `get_current_result_set_col_specs` which
always uses latest metadata. The only caller of old getter (some
integration test) is adjusted.

Author: Lorak-mmk

scylla/src/statement/prepared.rs

@@ -541,6 +541,13 @@ impl PreparedStatement {
         ColumnSpecs::new(self.shared.initial_result_metadata.col_specs())
     }
 
+    /// Access column specifications of the result set returned after the execution of this statement
+    pub fn get_current_result_set_col_specs(&self) -> ColumnSpecsGuard {
+        ColumnSpecsGuard {
+            result: self.shared.current_result_metadata.load(),
+        }
+    }
+
     /// Get the name of the partitioner used for this statement.
     pub fn get_partitioner_name(&self) -> &PartitionerName {
         &self.partitioner_name

scylla/tests/integration/statements/prepared.rs

@@ -474,7 +474,8 @@ async fn test_prepared_statement_col_specs() {
     ];
     assert_eq!(variable_col_specs, expected_variable_col_specs);
 
-    let result_set_col_specs = prepared.get_result_set_col_specs().as_slice();
+    let col_specs_guard = prepared.get_current_result_set_col_specs();
+    let result_set_col_specs = col_specs_guard.get().as_slice();
     let expected_result_set_col_specs = &[
         spec("k1", ColumnType::Native(NativeType::Int)),
         spec("k2", ColumnType::Native(NativeType::Varint)),