Add grace period for metrics late for aggregation (#6049)

docs/CONFIGURATION.md

@@ -325,6 +325,10 @@ Parameters that can be used with any aggregator plugin:
   how long for aggregators to wait before receiving metrics from input
   plugins, in the case that aggregators are flushing and inputs are gathering
   on the same interval.
+- **grace**: The duration when the metrics will still be aggregated
+  by the plugin, even though they're outside of the aggregation period. This
+  is needed in a situation when the agent is expected to receive late metrics
+  and it's acceptable to roll them up into next aggregation period.
 - **drop_original**: If true, the original metric will be dropped by the
   aggregator and will not get sent to the output plugins.
 - **name_override**: Override the base name of the measurement.  (Default is