chore: add deprecation notice for rollout_phase in docs (argoproj#2377)…

… (argoproj#2378)

Signed-off-by: iam-veeramalla <abhishek.veeramalla@gmail.com>

Signed-off-by: iam-veeramalla <abhishek.veeramalla@gmail.com>

docs/features/controller-metrics.md

@@ -50,7 +50,7 @@ The Argo Rollouts controller publishes the following prometheus metrics about Ar
 | `rollout_info`                      | Information about rollout. |
 | `rollout_info_replicas_available`   | The number of available replicas per rollout. |
 | `rollout_info_replicas_unavailable` | The number of unavailable replicas per rollout. |
-| `rollout_phase`                     | Information on the state of the rollout. |
+| `rollout_phase`                     | [**DEPRECATED - use rollout_info**] Information on the state of the rollout. |
 | `rollout_reconcile`                 | Rollout reconciliation performance. |
 | `rollout_reconcile_error`           | Error occurring during the rollout. |
 | `experiment_info`                   | Information about Experiment. |