Propagate traffic_rule label to custom metrics when the router timeout is reached

[krithika369]

A traffic rule label (traffic_rule) has been introduced in #280, which captures the selected rule on the mlp_turing_route_request_duration_ms and mlp_turing_turing_comp_request_duration_ms metrics, when the router has traffic rules enabled.

However, in a scenario where the overall timeout is reached on the router, the mlp_turing_turing_comp_request_duration_ms failure metric will not always capture the traffic_rule label. This is because, during context cancellation, the parent Fiber components may return first without waiting for the child component(s) to return and thus, the label set in the nested components may not be propagated upstream.