Merge branch 'docs-extract-kas-administration-docs' into 'master'

Move KAS installation under admin docs

See merge request gitlab-org/gitlab!57162

doc/administration/clusters/kas.md

+---
+stage: Configure
+group: Configure
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+---
+
+# Install the Kubernetes Agent Server (KAS) **(PREMIUM SELF)**
+
+The Kubernetes Agent Server (KAS) is a GitLab backend service dedicated to
+managing [Kubernetes Agents](../../user/clusters/agent/index.md).
+
+The KAS is already installed and available in GitLab.com under `wss://kas.gitlab.com`.
+See [how to use GitLab.com's KAS](../../user/clusters/agent/index.md#set-up-the-kubernetes-agent-server).
+This document describes how to install a KAS for GitLab self-managed instances.
+
+## Installation options
+
+As a GitLab administrator of self-managed instances, you can install KAS according to your GitLab
+installation method:
+
+- For [Omnibus installations](#install-kas-with-omnibus).
+- For [GitLab Helm Chart installations](#install-kas-with-the-gitlab-helm-chart).
+
+You can also opt to use an [external KAS](#use-an-external-kas-installation).
+
+### Install KAS with Omnibus
+
+For [Omnibus](https://docs.gitlab.com/omnibus/) package installations:
+
+1. Edit `/etc/gitlab/gitlab.rb` to enable the Kubernetes Agent Server:
+
+   ```ruby
+   gitlab_kas['enable'] = true
+   ```
+
+1. [Reconfigure GitLab](../restart_gitlab.md#omnibus-gitlab-reconfigure).
+
+To configure any additional options related to your KAS,
+refer to the **Enable GitLab KAS** section of the
+[`gitlab.rb.template`](https://gitlab.com/gitlab-org/omnibus-gitlab/-/blob/master/files/gitlab-config-template/gitlab.rb.template).
+
+### Install KAS with the GitLab Helm Chart
+
+For GitLab [Helm Chart](https://docs.gitlab.com/charts/)
+installations, you must set `global.kas.enabled` to `true`.
+For example, in a shell with `helm` and `kubectl`
+installed, run:
+
+```shell
+helm repo add gitlab https://charts.gitlab.io/
+helm repo update
+helm upgrade --install gitlab gitlab/gitlab \
+  --timeout 600s \
+  --set global.hosts.domain=<YOUR_DOMAIN> \
+  --set global.hosts.externalIP=<YOUR_IP> \
+  --set certmanager-issuer.email=<YOUR_EMAIL> \
+  --set global.kas.enabled=true # <-- without this, KAS will not be installed
+```
+
+To configure KAS, use a `gitlab.kas` sub-section in your `values.yaml` file:
+
+```yaml
+gitlab:
+  kas:
+    # put your KAS custom options here
+```
+
+For details, see [how to use the GitLab-KAS chart](https://docs.gitlab.com/charts/charts/gitlab/kas/).
+
+### Use an external KAS installation
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/299850) in GitLab 13.10.
+
+Besides installing KAS with GitLab, you can opt to configure GitLab to use an external KAS.
+
+For GitLab instances installed through the GitLab Helm Chart, see [how to configure your external KAS](https://docs.gitlab.com/charts/charts/globals.html#external-kas).
+
+For GitLab instances installed through Omnibus packages:
+
+1. Edit `/etc/gitlab/gitlab.rb` adding the paths to your external KAS:
+
+   ```ruby
+   gitlab_kas['enable'] = false
+   gitlab_kas['api_secret_key'] = 'Your shared secret between GitLab and KAS'
+
+   gitlab_rails['gitlab_kas_enabled'] = true
+   gitlab_rails['gitlab_kas_external_url'] = 'wss://kas.gitlab.example.com' # User-facing URL for the in-cluster agentk
+   gitlab_rails['gitlab_kas_internal_url'] = 'grpc://kas.internal.gitlab.example.com' # Internal URL for the GitLab backend
+   ```
+
+1. [Reconfigure GitLab](../restart_gitlab.md#omnibus-gitlab-reconfigure).
+
+## Troubleshooting
+
+If you face any issues with KAS, you can read the service logs
+with the following command:
+
+```shell
+kubectl logs -f -l=app=kas -n <YOUR-GITLAB-NAMESPACE>
+```
+
+In Omnibus GitLab, find the logs in `/var/log/gitlab/gitlab-kas/`.
+
+See also the [user documentation](../../user/clusters/agent/index.md#troubleshooting)
+for troubleshooting problems with individual agents.
+
+### KAS logs - GitOps: failed to get project info
+
+If you get the following error message:
+
+```json
+{"level":"warn","time":"2020-10-30T08:37:26.123Z","msg":"GitOps: failed to get project info","agent_id":4,"project_id":"root/kas-manifest001","error":"error kind: 0; status: 404"}
+```
+
+It means that the specified manifest project `root/kas-manifest001`
+doesn't exist or the manifest project is private. To fix it, make sure the project path is correct
+and its visibility is [set to public](../../public_access/public_access.md).
+
+### KAS logs - Configuration file not found
+
+If you get the following error message:
+
+```plaintext
+time="2020-10-29T04:44:14Z" level=warning msg="Config: failed to fetch" agent_id=2 error="configuration file not found: \".gitlab/agents/test-agent/config.yaml\
+```
+
+It means that the path to the configuration project is incorrect,
+or the path to `config.yaml` inside the project is not valid.
+
+To fix this, ensure that the paths to the configuration repo and to the `config.yaml` file
+are correct.