docs: try to explain the metric operation_duration_seconds better (#…

…12282)

Signed-off-by: Alan Clucas <alan@clucas.org>

docs/metrics.md

@@ -90,7 +90,7 @@ Number of API requests sent to the Kubernetes API.
 
 #### `argo_workflows_operation_duration_seconds`
 
-A histogram of durations of operations.
+A histogram of durations of operations. An operation is a single workflow reconciliation loop within the workflow-controller. It's the time for the controller to process a single workflow after it has been read from the cluster and is a measure of the performance of the controller affected by the complexity of the workflow.
 
 #### `argo_workflows_pods_count`
 
@@ -344,7 +344,7 @@ metricsConfig: |
 
   # Path is the path where metrics are emitted. Must start with a "/". Default is "/metrics"
   path: /metrics
-  
+
   # Port is the port where metrics are emitted. Default is "9090"
   port: 8080