feat: add session source as otel metadata tag (#9720)

Add session.source and user.account_id as global OTEL metric tags to
identify client surface and user.

codex-rs/otel/tests/suite/manager_metrics.rs

@@ -55,6 +55,7 @@ fn manager_attaches_metadata_tags_to_metrics() -> Result<()> {
         ("auth_mode".to_string(), AuthMode::ApiKey.to_string()),
         ("model".to_string(), "gpt-5.1".to_string()),
         ("service".to_string(), "codex-cli".to_string()),
+        ("session_source".to_string(), "cli".to_string()),
         ("source".to_string(), "tui".to_string()),
     ]);
     assert_eq!(attrs, expected);