Function merge_timer_stats 

pub fn merge_timer_stats(
    target: &mut SystemdUnitStats,
    collected: SystemdUnitStats,
    elapsed_ms: f64,
)

Merge D-Bus-collected timer stats into varlink-collected unit stats.

The varlink metrics API does not expose timer properties, so both the host (lib.rs) and container (machines.rs) varlink paths backfill them via collect_all_timers_dbus and merge the result here.

elapsed_ms is the wall time of that backfill. It is accounted the same way the D-Bus path accounts its own timer work — duration into per_unit_loop_ms and one timer_dbus_fetches per timer resolved — so the inner timings stay comparable between the two paths instead of under-reporting the varlink one.