server: stats endpoint (#468)

fixes #389

api/runner/stats.go

@@ -0,0 +1,46 @@
+package runner
+
+import "sync"
+
+type stats struct {
+	mu       sync.Mutex
+	queue    uint64
+	running  uint64
+	complete uint64
+}
+
+type Stats struct {
+	Queue    uint64
+	Running  uint64
+	Complete uint64
+}
+
+func (s *stats) Enqueue() {
+	s.mu.Lock()
+	s.queue++
+	s.mu.Unlock()
+}
+
+func (s *stats) Start() {
+	s.mu.Lock()
+	s.queue--
+	s.running++
+	s.mu.Unlock()
+}
+
+func (s *stats) Complete() {
+	s.mu.Lock()
+	s.running--
+	s.complete++
+	s.mu.Unlock()
+}
+
+func (s *stats) Stats() Stats {
+	var stats Stats
+	s.mu.Lock()
+	stats.Running = s.running
+	stats.Complete = s.complete
+	stats.Queue = s.queue
+	s.mu.Unlock()
+	return stats
+}