Add metric for builds waiting for download slot

(cherry picked from commit f23ec71227911891807706b6b978836e4d80edde)

src/hydra-queue-runner/state.hh

@@ -376,6 +376,7 @@ private:
     counter nrStepsDone{0};
     counter nrStepsBuilding{0};
     counter nrStepsCopyingTo{0};
+    counter nrStepsWaitingForDownloadSlot{0};
     counter nrStepsCopyingFrom{0};
     counter nrStepsWaiting{0};
     counter nrUnsupportedSteps{0};