config :hello, Hello.Repo,
  username: System.get_env("POSTGRES_USER", "postgres"),
  password: System.get_env("POSTGRES_PASSWORD", "postgres"),
  database: "hello_dev",
  hostname: "/cloudsql/<project_name>:<region>:<instance_name>,
  pool_size: 10

metadata {
   annotations = {
      "run.googleapis.com/cloudsql-instances" = google_sql_database_instance.postgres_instance.connection_name
      ...
   }
}

And in the direct deployment from gcloud console, my service account was different so i was missing the permissions to connect to cloud sql.