feat(dashboards): Add feature flag for metrics in AM2+ (#78870)

Since users transitioning to AM3 will see full ingestion of
transactions, orgs with highly sampled rates will see a large spike due
to us showing transactions data instead of metrics data.

Add a feature flag so we can continue showing metrics data until the
transactions are fully ingested for at least 90d for orgs going onto
AM3.

Author: narsaynorath

src/sentry/features/temporary.py

@@ -98,6 +98,8 @@ def register_temporary_features(manager: FeatureManager):
     manager.add("organizations:dashboards-import", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=True)
     # Enable metrics enhanced performance in dashboards
     manager.add("organizations:dashboards-mep", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=True)
+    # Enable metrics enhanced performance for AM2+ customers as they transition from AM2 to AM3
+    manager.add("organizations:dashboards-metrics-transition", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=True)
     manager.add("organizations:dashboards-span-metrics", OrganizationFeature, FeatureHandlerStrategy.OPTIONS, api_expose=False)
     # Enable releases overlay on dashboard chart widgets
     manager.add("organizations:dashboards-releases-on-charts", OrganizationFeature, FeatureHandlerStrategy.FLAGPOLE, api_expose=True)