[downloader] Add average speed to final progress line

Fixes: https://github.com/ytdl-org/youtube-dl/issues/31122

yt_dlp/downloader/common.py

@@ -335,7 +335,10 @@ class FileDownloader:
         if s['status'] == 'finished':
             if self.params.get('noprogress'):
                 self.to_screen('[download] Download completed')
+            speed = try_call(lambda: s['total_bytes'] / s['elapsed'])
             s.update({
+                'speed': speed,
+                '_speed_str': self.format_speed(speed).strip(),
                 '_total_bytes_str': format_bytes(s.get('total_bytes')),
                 '_elapsed_str': self.format_seconds(s.get('elapsed')),
                 '_percent_str': self.format_percent(100),
@@ -344,6 +347,7 @@ class FileDownloader:
                 '100%%',
                 with_fields(('total_bytes', 'of %(_total_bytes_str)s')),
                 with_fields(('elapsed', 'in %(_elapsed_str)s')),
+                with_fields(('speed', 'at %(_speed_str)s')),
                 delim=' '))
 
         if s['status'] != 'downloading':