make stalebot ignore prs tagged "meta"

.github/stale.yml

@@ -10,6 +10,7 @@ daysUntilClose: 15
 # Issues or Pull Requests with these labels will never be considered stale. Set to `[]` to disable
 exemptLabels:
   - on hold
+  - meta
 
 # Set to true to ignore issues in a project (defaults to false)
 exemptProjects: false