Add FLECS_METRICS to list of addons in quickstart

docs/Quickstart.md

@@ -111,6 +111,7 @@ Addon         | Description                                      | Define
 [Snapshot](/flecs/group__c__addons__snapshot.html)         | Take snapshots of the world & restore them       | FLECS_SNAPSHOT      |
 [Stats](/flecs/group__c__addons__stats.html)               | See what's happening in a world with statistics  | FLECS_STATS         |
 [Monitor](/flecs/group__c__addons__monitor.html)           | Periodically collect & store statistics          | FLECS_MONITOR       |
+[Metrics](/flecs/group__c__addons__metrics.html)           | Create metrics from user-defined components      | FLECS_METRICS       |
 [Log](/flecs/group__c__addons__log.html)                   | Extended tracing and error logging               | FLECS_LOG           |
 [Journal](/flecs/group__c__addons__journal.html)           | Journaling of API functions                      | FLECS_JOURNAL       |
 [App](/flecs/group__c__addons__app.html)                   | Flecs application framework                      | FLECS_APP           |