Add prometheus support (#950)

Fixes #888

src/rpc/WorkQueue.cpp

@@ -21,13 +21,35 @@
 
 namespace rpc {
 
-WorkQueue::WorkQueue(std::uint32_t numWorkers, uint32_t maxSize) : ioc_{numWorkers}
+WorkQueue::WorkQueue(std::uint32_t numWorkers, uint32_t maxSize)
+    : queued_{PrometheusService::counterInt(
+          "work_queue_queued_total_number",
+          util::prometheus::Labels(),
+          "The total number of tasks queued for processing"
+      )}
+    , durationUs_{PrometheusService::counterInt(
+          "work_queue_cumulitive_tasks_duration_us",
+          util::prometheus::Labels(),
+          "The total number of microseconds tasks were waiting to be executed"
+      )}
+    , curSize_{PrometheusService::gaugeInt(
+          "work_queue_current_size",
+          util::prometheus::Labels(),
+          "The current number of tasks in the queue"
+      )}
+    , ioc_{numWorkers}
 {
     if (maxSize != 0)
         maxSize_ = maxSize;
 }
 
 WorkQueue::~WorkQueue()
+{
+    join();
+}
+
+void
+WorkQueue::join()
 {
     ioc_.join();
 }