update monitoring doc

Signed-off-by: Ziqi Zhao <[email protected]>

docs/monitoring.md

@@ -34,9 +34,11 @@ Most exporters offer a [queue/retry mechanism](../exporter/exporterhelper/README
 that is recommended as the retry mechanism for the Collector and as such should
 be used in any production deployment.
 
-**TODO:** Add metric to monitor queue length.
+The `otelcol_exporter_queue_capacity` indicates the capacity of the retry queue (in batches). The `otelcol_exporter_queue_size` indicates the current size of retry queue. So you can check those two metrics to see if queue size is enough for you load. 
 
-Currently, the queue/retry mechanism only supports logging for monitoring. Check
+The `otelcol_exporter_enqueue_failed_spans`, `otelcol_exporter_enqueue_failed_metric_points` and `otelcol_exporter_enqueue_failed_log_records` indicate the number of span/metric points/log records failed to be added to the sending queue. This maybe cause by the queue is full of unsettled elements so you should decrease your sending rate or horizontally scale your collector.
+
+The queue/retry mechanism also supports logging for monitoring. Check
 the logs for messages like `"Dropping data because sending_queue is full"`.
 
 ### Receive Failures