Improve metadata for `SkipCheckIfFeeless` (#7813)

If the inner transaction extension used inside `SkipCheckIfFeeless` are
multiples then the metadata is not correct, it is now fixed.

E.g. if the transaction extension is `SkipCheckIfFeeless::<Runtime,
(Payment1, Payment2)>` then the metadata was wrong.

---------

Co-authored-by: cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Bastian Köcher <git@kchr.de>

prdoc/pr_7813.prdoc

+title: Improve metadata for `SkipCheckIfFeeless`
+doc:
+- audience: Runtime Dev
+  description: |-
+    If the inner transaction extension used inside `SkipCheckIfFeeless` are multiples then the metadata is not correct, it is now fixed.
+
+    E.g. if the transaction extension is `SkipCheckIfFeeless::<Runtime, (Payment1, Payment2)>` then the metadata was wrong.
+crates:
+- name: pallet-skip-feeless-payment
+  bump: patch

substrate/frame/transaction-payment/skip-feeless-payment/src/lib.rs

@@ -36,6 +36,8 @@
 
 #![cfg_attr(not(feature = "std"), no_std)]
 
+extern crate alloc;
+
 use codec::{Decode, DecodeWithMemTracking, Encode};
 use frame_support::{
 	dispatch::{CheckIfFeeless, DispatchResult},
@@ -129,6 +131,10 @@ where
 	const IDENTIFIER: &'static str = S::IDENTIFIER;
 	type Implicit = S::Implicit;
 
+	fn metadata() -> alloc::vec::Vec<sp_runtime::traits::TransactionExtensionMetadata> {
+		S::metadata()
+	}
+
 	fn implicit(&self) -> Result<Self::Implicit, TransactionValidityError> {
 		self.0.implicit()
 	}

substrate/frame/transaction-payment/skip-feeless-payment/src/tests.rs

@@ -94,3 +94,18 @@ fn validate_prepare_works() {
 	assert_eq!(ValidateCount::get(), 2);
 	assert_eq!(PrepareCount::get(), 2);
 }
+
+#[test]
+fn metadata_for_wrap_multiple_tx_ext() {
+	let metadata = SkipCheckIfFeeless::<Runtime, (DummyExtension, DummyExtension)>::metadata();
+	let mut expected_metadata = vec![];
+	expected_metadata.extend(DummyExtension::metadata().into_iter());
+	expected_metadata.extend(DummyExtension::metadata().into_iter());
+
+	assert_eq!(metadata.len(), expected_metadata.len());
+	for i in 0..expected_metadata.len() {
+		assert_eq!(metadata[i].identifier, expected_metadata[i].identifier);
+		assert_eq!(metadata[i].ty, expected_metadata[i].ty);
+		assert_eq!(metadata[i].implicit, expected_metadata[i].implicit);
+	}
+}