queue-runner: add prom metrics to allow detecting internal bottlenecks

By looking at the ratio of running vs. waiting for the dispatcher and
the queue monitor, we should get better visibility into what hydra is
currently bottlenecked on.

There are other side effects we can try to measure to get to the same
result, but having a simple way doesn't cost us much.

src/hydra-queue-runner/hydra-queue-runner.cc

@@ -77,6 +77,34 @@ State::PromMetrics::PromMetrics()
             .Register(*registry)
             .Add({})
     )
+    , dispatcher_time_spent_running(
+        prometheus::BuildCounter()
+            .Name("hydraqueuerunner_dispatcher_time_spent_running")
+            .Help("Time (in micros) spent running the dispatcher")
+            .Register(*registry)
+            .Add({})
+    )
+    , dispatcher_time_spent_waiting(
+        prometheus::BuildCounter()
+            .Name("hydraqueuerunner_dispatcher_time_spent_waiting")
+            .Help("Time (in micros) spent waiting for the dispatcher to obtain work")
+            .Register(*registry)
+            .Add({})
+    )
+    , queue_monitor_time_spent_running(
+        prometheus::BuildCounter()
+            .Name("hydraqueuerunner_queue_monitor_time_spent_running")
+            .Help("Time (in micros) spent running the queue monitor")
+            .Register(*registry)
+            .Add({})
+    )
+    , queue_monitor_time_spent_waiting(
+        prometheus::BuildCounter()
+            .Name("hydraqueuerunner_queue_monitor_time_spent_waiting")
+            .Help("Time (in micros) spent waiting for the queue monitor to obtain work")
+            .Register(*registry)
+            .Add({})
+    )
 {
 
 }