feat(mqtt sink): expose retain config flag

src/sinks/mqtt/config.rs

@@ -52,6 +52,10 @@ pub struct MqttSinkConfig {
     /// MQTT publish topic (templates allowed)
     pub topic: Template,
 
+    /// Whether the messages should be retained by the server
+    #[serde(default = "default_retain")]
+    pub retain: bool,
+
     #[configurable(derived)]
     pub encoding: EncodingConfig,
 
@@ -112,6 +116,10 @@ const fn default_qos() -> MqttQoS {
     MqttQoS::AtLeastOnce
 }
 
+const fn default_retain() -> bool {
+    false
+}
+
 impl Default for MqttSinkConfig {
     fn default() -> Self {
         Self {
@@ -124,6 +132,7 @@ impl Default for MqttSinkConfig {
             clean_session: default_clean_session(),
             tls: None,
             topic: Template::try_from("vector").expect("Cannot parse as a template"),
+            retain: default_retain(),
             encoding: JsonSerializerConfig::default().into(),
             acknowledgements: AcknowledgementsConfig::default(),
             quality_of_service: MqttQoS::default(),

src/sinks/mqtt/service.rs

@@ -53,6 +53,7 @@ impl MetaDescriptive for MqttRequest {
 pub(super) struct MqttService {
     pub(super) client: AsyncClient,
     pub(super) quality_of_service: MqttQoS,
+    pub(super) retain: bool,
 }
 
 #[derive(Debug, Snafu)]
@@ -72,13 +73,14 @@ impl Service<MqttRequest> for MqttService {
 
     fn call(&mut self, req: MqttRequest) -> Self::Future {
         let quality_of_service = self.quality_of_service;
+        let retain = self.retain;
         let client = self.client.clone();
 
         Box::pin(async move {
             let byte_size = req.body.len();
 
             let res = client
-                .publish(&req.topic, quality_of_service.into(), false, req.body)
+                .publish(&req.topic, quality_of_service.into(), retain, req.body)
                 .await;
             match res {
                 Ok(()) => Ok(MqttResponse {

src/sinks/mqtt/sink.rs

@@ -52,6 +52,7 @@ pub struct MqttSink {
     encoder: Encoder<()>,
     connector: MqttConnector,
     quality_of_service: MqttQoS,
+    retain: bool,
 }
 
 pub(super) struct MqttEvent {
@@ -70,6 +71,7 @@ impl MqttSink {
             encoder,
             connector,
             quality_of_service: config.quality_of_service,
+            retain: config.retain,
         })
     }
 
@@ -115,6 +117,7 @@ impl MqttSink {
         let service = ServiceBuilder::new().service(MqttService {
             client,
             quality_of_service: self.quality_of_service,
+            retain: self.retain,
         });
 
         let request_builder = MqttRequestBuilder {

website/cue/reference/components/sinks/base/mqtt.cue

@@ -325,6 +325,11 @@ base: components: sinks: mqtt: configuration: {
 			}
 		}
 	}
+	retain: {
+		description: "Whether the messages should be retained by the server"
+		required:    false
+		type: bool: default: false
+	}
 	tls: {
 		description: "Configures the TLS options for incoming/outgoing connections."
 		required:    false