Correctly compute process times during ProcessHealth::observe (#8793)

I believe I found a bug where during computation of `pid_process_seconds_total` we add `children_system` twice.


  I assume that it was originally intended to add `children_system` and `children_user` once each


Co-Authored-By: Mac L <mjladson@pm.me>

common/health_metrics/src/observe.rs

@@ -121,7 +121,7 @@ impl Observe for ProcessHealth {
             pid_mem_shared_memory_size: process_mem.shared(),
             pid_process_seconds_total: process_times.busy().as_secs()
                 + process_times.children_system().as_secs()
-                + process_times.children_system().as_secs(),
+                + process_times.children_user().as_secs(),
         })
     }
 }