Fix missing GatewayStats v4_migrate.

This is needed as the gateway_id field has changed from bytes to string
(as the Protobuf JSON mapping encodes bytes to base64).

api/rust/src/gw.rs

@@ -277,3 +277,11 @@ impl DownlinkTxAck {
         }
     }
 }
+
+impl GatewayStats {
+    pub fn v4_migrate(&mut self) {
+        if self.gateway_id.is_empty() {
+            self.gateway_id = hex::encode(&self.gateway_id_legacy);
+        }
+    }
+}

chirpstack/src/gateway/backend/mqtt.rs

@@ -328,6 +328,7 @@ async fn message_callback(region_name: &str, region_common_name: CommonName, msg
                 })
                 .inc();
             let mut event = chirpstack_api::gw::GatewayStats::decode(&mut Cursor::new(b))?;
+            event.v4_migrate();
             event
                 .meta_data
                 .insert("region_name".to_string(), region_name.to_string());