Add placement table api doc (#3631)

* add placement table api doc

Signed-off-by: zhangchao <zchao9100@gmail.com>

* typo fix

Signed-off-by: zhangchao <zchao9100@gmail.com>

* fix review

Signed-off-by: zhangchao <zchao9100@gmail.com>

* fix example

Signed-off-by: zhangchao <zchao9100@gmail.com>

---------

Signed-off-by: zhangchao <zchao9100@gmail.com>
Co-authored-by: Mark Fussell <markfussell@gmail.com>
Co-authored-by: Hannah Hunter <94493363+hhunter-ms@users.noreply.github.com>

Author: 3 people

daprdocs/content/en/concepts/dapr-services/placement.md

@@ -14,3 +14,72 @@ The placement service Docker container is started automatically as part of [`dap
 ## Kubernetes mode
 
 The placement service is deployed as part of `dapr init -k`, or via the Dapr Helm charts. For more information on running Dapr on Kubernetes, visit the [Kubernetes hosting page]({{< ref kubernetes >}}).
+
+## Placement tables
+
+There is an HTTP API `/placement/state` for placement service that exposes placement table information. The API is exposed on the sidecar on the same port as the healthz. This is an unauthenticated endpoint, and is disabled by default. You need to set `DAPR_PLACEMENT_METADATA_ENABLED` environment or `metadata-enabled` command line args to true to enable it. If you are using helm you just need to set `dapr_placement.metadataEnabled` to true.
+
+### Usecase:
+The placement table API can be used for retrieving the current placement table, which contains all the actors registered. This can be helpful for debugging and allows tools to extract and present information about actors.
+
+### HTTP Request
+
+```
+GET http://localhost:<healthzPort>/placement/state
+```
+
+### HTTP Response Codes
+
+Code | Description
+---- | -----------
+200  | Placement tables information returned
+500  | Placement could not return the placement tables information
+
+### HTTP Response Body
+
+**Placement tables API Response Object**
+
+Name                   | Type                                                                  | Description
+----                   | ----                                                                  | -----------
+tableVersion           | int                                                                   | The placement table version
+hostList               | [Actor Host Info](#actorhostinfo)[]                                   | A json array of registered actors host info.
+
+<a id="actorhostinfo"></a>**Actor Host Info**
+
+Name  | Type    | Description
+----  | ----    | -----------
+name  | string  | The host:port address of the actor.
+appId | string  | app id.
+actorTypes | json string array | List of actor types it hosts.
+updatedAt | timestamp | Timestamp of the actor registered/updated.
+
+### Examples
+
+```shell
+ curl localhost:8080/placement/state
+```
+
+```json
+{
+	"hostList": [{
+			"name": "198.18.0.1:49347",
+			"appId": "actor1",
+			"actorTypes": ["testActorType1", "testActorType3"],
+			"updatedAt": 1690274322325260000
+		},
+		{
+			"name": "198.18.0.2:49347",
+			"appId": "actor2",
+			"actorTypes": ["testActorType2"],
+			"updatedAt": 1690274322325260000
+		},
+		{
+			"name": "198.18.0.3:49347",
+			"appId": "actor2",
+			"actorTypes": ["testActorType2"],
+			"updatedAt": 1690274322325260000
+		}
+	],
+	"tableVersion": 1
+}
+```