Add API docs for sys/monitor (#9968)

website/data/api-navigation.js

@@ -119,6 +119,7 @@ export default [
         category: 'mfa',
         content: ['duo', 'okta', 'pingid', 'totp'],
       },
+      'monitor',
       'mounts',
       'namespaces',
       'plugins-reload-backend',

website/pages/api-docs/system/monitor.mdx

@@ -0,0 +1,42 @@
+---
+layout: api
+page_title: /sys/monitor - HTTP API
+sidebar_title: <code>/sys/monitor</code>
+description: The `/sys/monitor` endpoint is used to receive streaming logs from the Vault server.
+---
+
+# `/sys/monitor`
+
+The `/sys/monitor` endpoint is used to receive streaming logs from the Vault server.
+
+## Monitor system logs
+
+This endpoint streams logs back to the client from Vault. Note that unlike most API endpoints in Vault, this one
+does not return JSON by default. This will send back data in whatever log format Vault has been configured with. By
+default, this is text.
+
+| Method | Path          |
+| :----- | :------------ |
+| `GET`  | `/sys/monitor` |
+
+### Parameters
+
+- `log_level` `(string: "info")` – Specifies the log level to use when streaming logs. This defaults to `info`
+  if not specified.
+
+### Sample Request
+
+```shell-session
+$ curl \
+    --header "X-Vault-Token: ..." \
+    http://127.0.0.1:8200/v1/sys/monitor?log_level=debug"
+```
+
+### Sample Response
+
+```
+2020-09-15T11:28:09.188-0700 [INFO]  core: successful mount: namespace= path=foo/ type=kv
+2020-09-15T11:28:18.265-0700 [DEBUG] core.secrets.deletion: clearing view: namespace=root path=foo/ total_keys=0
+2020-09-15T11:28:18.265-0700 [DEBUG] core.secrets.deletion: view cleared: namespace=root path=foo/
+2020-09-15T11:28:18.265-0700 [INFO]  core: successfully unmounted: path=foo/ namespace=
+```