Enable the kubernetes agent internal api by default

Since we now have the JWT internal auth since bf5b5cf9, we don't
need the protection of FF anymore. However we still keep the FF so that
we can disable this if necessary.

Adds docs for GitLab Kubernetes agent internal API

changelogs/unreleased/ff-kubernetes_agent_internal_api.yml

+---
+title: Internal API for GitLab Kubernetes agent
+merge_request: 41045
+author:
+type: other

doc/development/internal_api.md

@@ -370,3 +370,64 @@ Example response:
   "reference_counter_decreased": true
 }
 ```
+
+## Kubernetes agent information
+
+Called from GitLab Kubernetes Agent Server (kas) to retrieve agent
+information for the given agent token. This returns the Gitaly connection
+information for the agent's project in order for kas to fetch and update
+the agent's configuration.
+
+```plaintext
+GET /internal/kubernetes/agent_info
+```
+
+Example Request:
+
+```shell
+curl --request GET --header "Gitlab-Kas-Api-Request: <JWT token>" --header "Authorization: Bearer <agent token>" "http://localhost:3000/api/v4/internal/kubernetes/agent_info"
+```
+
+## Kubernetes agent project information
+
+Called from GitLab Kubernetes Agent Server (kas) to retrieve project
+information for the given agent token. This returns the Gitaly
+connection for the requested project. GitLab kas uses this to configure
+the agent to fetch Kubernetes resources from the project repository to
+sync.
+
+Only public projects are currently supported. For private projects, the ability for the
+agent to be authorized is [not yet implemented](https://gitlab.com/gitlab-org/gitlab/-/issues/220912).
+
+| Attribute | Type   | Required | Description |
+|:----------|:-------|:---------|:------------|
+| `id` | integer/string | yes | The ID or [URL-encoded path of the project](../api/README.md#namespaced-path-encoding) |
+
+```plaintext
+GET /internal/kubernetes/project_info
+```
+
+Example Request:
+
+```shell
+curl --request GET --header "Gitlab-Kas-Api-Request: <JWT token>" --header "Authorization: Bearer <agent token>" "http://localhost:3000/api/v4/internal/kubernetes/project_info?id=7"
+```
+
+## Kubernetes agent usage metrics
+
+Called from GitLab Kubernetes Agent Server (kas) to increase the usage
+metric counters.
+
+| Attribute | Type   | Required | Description |
+|:----------|:-------|:---------|:------------|
+| `gitops_sync_count` | integer| yes | The number to increase the `gitops_sync_count` counter by |
+
+```plaintext
+POST /internal/kubernetes/usage_metrics
+```
+
+Example Request:
+
+```shell
+curl --request POST --header "Gitlab-Kas-Api-Request: <JWT token>" --header "Content-Type: application/json" --data '{"gitops_sync_count":1}' "http://localhost:3000/api/v4/internal/kubernetes/usage_metrics"
+```

lib/api/internal/kubernetes.rb

@@ -45,7 +45,7 @@ module API
         end
 
         def check_feature_enabled
-          not_found! unless Feature.enabled?(:kubernetes_agent_internal_api)
+          not_found! unless Feature.enabled?(:kubernetes_agent_internal_api, default_enabled: true)
         end
 
         def check_agent_token