src: initialize prev_idle_time_ on ThreadMetrics

Make sure `prev_idle_time_` is initialized in the
`ThreadMetrics::ThreadMetrics(SharedEnvInst envinst)` constructor
so the loop utilization calculations are correct, otherwise the default
value 0 is used which may lead to meaningless huge values (greater than
1 for the matter).
No need to initialize this value in the
`ThreadMetrics::ThreadMetrics(uint64_t thread_id)` as this is only used
in the `EnvInst` constructor on which having a value of 0 makes sense.

src/nsolid.cc

@@ -285,6 +285,7 @@ ThreadMetrics::ThreadMetrics(SharedEnvInst envinst)
   stor_.thread_id = thread_id_;
   stor_.prev_call_time_ = uv_hrtime();
   stor_.current_hrtime_ = stor_.prev_call_time_;
+  stor_.prev_idle_time_ = uv_metrics_idle_time(envinst->event_loop());
 }
 
 
@@ -300,6 +301,8 @@ ThreadMetrics::ThreadMetrics(uint64_t thread_id)
   stor_.thread_id = thread_id_;
   stor_.prev_call_time_ = uv_hrtime();
   stor_.current_hrtime_ = stor_.prev_call_time_;
+  // Letting prev_idle_time_ be 0 is ok as this is only used by the EnvInst
+  // constructor.
 }
 
 

test/addons/nsolid-env-metrics/binding.cc

@@ -16,6 +16,7 @@ struct Metrics {
 
 static void got_env_metrics(SharedThreadMetrics tm_sp, Metrics* m) {
   assert(tm_sp == m->tm);
+  assert(tm_sp->Get().loop_utilization <= 1);
   cb_cntr++;
   delete m;
 }

test/addons/nsolid-env-metrics/nsolid-env-metrics.js

@@ -37,4 +37,8 @@ for (let i = 0; i < 10; i++) {
   }));
 }
 
-binding.getMetrics();
+// Let then main thread be a bit idle so we can check that loop_utilization is
+// correctly calculdate.
+setTimeout(() => {
+  binding.getMetrics();
+}, 100);