Add documentation for environment tier

This commit adds the documentation for environment tier
feature that was added in GitLab 13.10.

app/models/environment.rb

@@ -461,8 +461,6 @@ def generate_slug
   end
   def ensure_environment_tier
-    return unless ::Feature.enabled?(:environment_tier, project, default_enabled: :yaml)
-
     self.tier ||= guess_tier
   end

app/services/deployments/update_environment_service.rb

@@ -68,7 +68,7 @@ def renew_auto_stop_in
     end
     def renew_deployment_tier
-      return unless deployable && ::Feature.enabled?(:environment_tier, deployable.project, default_enabled: :yaml)
+      return unless deployable
       if (tier = deployable.environment_deployment_tier)
         environment.tier = tier

changelogs/unreleased/docs-environment-tier.yml

+---
+title: Support environment deployment tier
+merge_request: 56081
+author:
+type: added

config/feature_flags/development/environment_tier.yml

----
-name: environment_tier
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/55471
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/323166
-milestone: '13.10'
-type: development
-group: group::release
-default_enabled: false

doc/ci/environments/index.md

@@ -123,6 +123,30 @@ Some variables cannot be used as environment names or URLs.
 For more information about the `environment` keywords, see
 [the `.gitlab-ci.yml` keyword reference](../yaml/README.md#environment).
+## Deployment tier of environments (**FREE**)
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/300741) in GitLab 13.10.
+
+There are cases where you might want to use a code name as an environment name instead of using
+an [industry standard](https://en.wikipedia.org/wiki/Deployment_environment). For example, your environment might be called `customer-portal` instead of `production`.
+This is perfectly fine, however, it loses information that the specific
+environment is used as production.
+
+To keep information that a specific environment is for production or
+some other use, you can set one of the following tiers to each environment:
+
+| Environment tier           | Environment names examples                                |
+| ----           | --------                                |
+| `production`   | Production, Live                        |
+| `staging`      | Staging, Model, Pre, Demo               |
+| `testing`      | Test, QC                                |
+| `development`  | Dev, [Review apps](../review_apps/index.md), Trunk |
+| `other`        |                                         |
+
+By default, an approximate tier is automatically guessed and set from [the environment name](../yaml/README.md#environmentname).
+Alternatively, you can specify a specific tier with `deployment_tier` keyword,
+see the [`.gitlab-ci.yml` syntax reference](../yaml/README.md#environmentdeployment_tier) for more details.
+
 ## Configure manual deployments
 You can create a job that requires someone to manually start the deployment.

doc/ci/yaml/README.md

@@ -2632,6 +2632,23 @@ that are [managed by GitLab](../../user/project/clusters/index.md#gitlab-managed
 To follow progress on support for GitLab-managed clusters, see the
 [relevant issue](https://gitlab.com/gitlab-org/gitlab/-/issues/38054).
+#### `environment:deployment_tier`
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/27630) in GitLab 13.10.
+
+Use the `deployment_tier` keyword to specify the tier of the deployment environment:
+
+```yaml
+deploy:
+  script: echo
+  environment:
+    name: customer-portal
+    deployment_tier: production
+```
+
+For more information,
+see [Deployment tier of environments](../environments/index.md#deployment-tier-of-environments).
+
 #### Dynamic environments
 Use CI/CD [variables](../variables/README.md) to dynamically name environments.

lib/gitlab/ci/pipeline/seed/environment.rb

@@ -15,7 +15,7 @@ def to_resource
             environments.safe_find_or_create_by(name: expanded_environment_name) do |environment|
               # Initialize the attributes at creation
               environment.auto_stop_in = auto_stop_in
-              environment.tier = deployment_tier if ::Feature.enabled?(:environment_tier, job.project, default_enabled: :yaml)
+              environment.tier = deployment_tier
             end
           end

spec/lib/gitlab/ci/pipeline/seed/environment_spec.rb

@@ -123,16 +123,6 @@
             expect { subject }.to raise_error(ArgumentError, "'unknown' is not a valid tier")
           end
         end
-
-        context 'when environment_tier feature flag is disabled' do
-          before do
-            stub_feature_flags(environment_tier: false)
-          end
-
-          it 'does not set the specified tier' do
-            expect(subject.tier).to be_nil
-          end
-        end
       end
       context 'when environment has already been created' do

spec/models/environment_spec.rb

@@ -53,18 +53,6 @@
       expect { environment.update!(name: 'gstg') }.not_to change { environment.reload.tier }
     end
-
-    context 'when environment_tier feature flag is disabled' do
-      before do
-        stub_feature_flags(environment_tier: false)
-      end
-
-      it 'does not ensure environment tier' do
-        environment = build(:environment, name: 'gprd', tier: nil)
-
-        expect { environment.save }.not_to change { environment.tier }
-      end
-    end
   end
   describe '.order_by_last_deployed_at' do

spec/services/deployments/update_environment_service_spec.rb

@@ -168,16 +168,6 @@
               .to change { environment.reset.tier }.from(nil).to('other')
           end
         end
-
-        context 'when environment_tier feature flag is disabled' do
-          before do
-            stub_feature_flags(environment_tier: false)
-          end
-
-          it 'does not set the specified deployment tier' do
-            expect { subject.execute }.not_to change { environment.reset.tier }
-          end
-        end
       end
     end