Add metric for builds waiting for download slot

(cherry picked from commit f23ec71227911891807706b6b978836e4d80edde)

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

@@ -540,6 +540,7 @@ void State::buildRemote(ref<Store> destStore,
         /* Throttle CPU-bound work. Opportunistically skip updating the current
          * step, since this requires a DB roundtrip. */
         if (!localWorkThrottler.try_acquire()) {
+            MaintainCount<counter> mc(nrStepsWaitingForDownloadSlot);
             updateStep(ssWaitingForLocalSlot);
             localWorkThrottler.acquire();
         }

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

@@ -573,6 +573,7 @@ void State::dumpStatus(Connection & conn)
         {"nrActiveSteps", activeSteps_.lock()->size()},
         {"nrStepsBuilding", nrStepsBuilding.load()},
         {"nrStepsCopyingTo", nrStepsCopyingTo.load()},
+        {"nrStepsWaitingForDownloadSlot", nrStepsWaitingForDownloadSlot.load()},
         {"nrStepsCopyingFrom", nrStepsCopyingFrom.load()},
         {"nrStepsWaiting", nrStepsWaiting.load()},
         {"nrUnsupportedSteps", nrUnsupportedSteps.load()},

src/hydra-queue-runner/state.hh

@@ -369,6 +369,7 @@ private:
     counter nrStepsDone{0};
     counter nrStepsBuilding{0};
     counter nrStepsCopyingTo{0};
+    counter nrStepsWaitingForDownloadSlot{0};
     counter nrStepsCopyingFrom{0};
     counter nrStepsWaiting{0};
     counter nrUnsupportedSteps{0};