Fix presenting duplicate metrics

If the collection takes too long, a new collection can occur before the
preivous has finished. This causes duplicate metrics to be presented to
Prometheus.

Fix this by introducing a semaphore for collection.

Author: MoteHue

radosgw_usage_exporter.py

@@ -33,6 +33,7 @@ def __init__(
         self.store = store
         self.insecure = insecure
         self.timeout = timeout
+        self.collect_semaphore = False
 
         # helpers for default schema
         if not self.host.startswith("http"):
@@ -53,6 +54,12 @@ def collect(self):
             http://docs.ceph.com/docs/master/radosgw/adminops/#get-bucket-info
         """
 
+        if self.collect_semaphore:
+            logging.debug("Previous collection still in progress at time "
+                         f"{time.time()}, cancelling collection.")
+            return
+        self.collect_semaphore = True
+
         start = time.time()
         # setup empty prometheus metrics
         self._setup_empty_prometheus_metrics()
@@ -84,6 +91,8 @@ def collect(self):
         for metric in list(self._prometheus_metrics.values()):
             yield metric
 
+        self.collect_semaphore = False
+
     def _session(self):
         """
         Setup Requests connection settings.