Fix extrinsics count logging in frame-system (#4461)

The storage item ExtrinsicIndex is already taken before the `finalize()`
in `note_finished_extrinsics()`, rendering it's always 0 in the log.
This commit fixes it by using the proper API for extrinsics count.

---------

Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

prdoc/pr_4461.prdoc

@@ -0,0 +1,10 @@
+title: Fix extrinsics count logging in frame-system
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      Fixes the issue of the number of extrinsics in the block always being 0 in the log of frame-system.
+
+crates:
+  - name: frame-system
+    bump: patch 

substrate/frame/system/src/lib.rs

@@ -1780,7 +1780,7 @@ impl<T: Config> Pallet<T> {
 			"[{:?}] {} extrinsics, length: {} (normal {}%, op: {}%, mandatory {}%) / normal weight:\
 			 {} ({}%) op weight {} ({}%) / mandatory weight {} ({}%)",
 			Self::block_number(),
-			Self::extrinsic_index().unwrap_or_default(),
+			Self::extrinsic_count(),
 			Self::all_extrinsics_len(),
 			sp_runtime::Percent::from_rational(
 				Self::all_extrinsics_len(),