feat(helm): add documentation about metric args

This helps documenting this issue:
https://github.com/kubernetes/ingress-nginx/issues/8233

and relates to this documentation:
https://github.com/kubernetes/ingress-nginx/blob/main/docs/user-guide/monitoring.md#histogram-buckets

charts/ingress-nginx/values.yaml

@@ -170,6 +170,9 @@ controller:
   extraArgs: {}
   ## extraArgs:
   ##   default-ssl-certificate: "<namespace>/<secret_name>"
+  ##   time-buckets: "0.005,0.01,0.025,0.05,0.1,0.25,0.5,1,2.5,5,10
+  ##   length-buckets: "10,20,30,40,50,60,70,80,90,100"
+  ##   size-buckets: "10,100,1000,10000,100000,1e+06,1e+07"
 
   # -- Additional environment variables to set
   extraEnvs: []