feat: Add /tags endpoint

The instance tags are useful to understand things for debugging, such as:
- The launch template version
- The CloudFormation stack name

app/controllers/ManagementController.scala

@@ -4,12 +4,29 @@ import buildinfo.BuildInfo
 import play.api.Logging
 import play.api.libs.json.Json
 import play.api.mvc.{Action, AnyContent, BaseController, ControllerComponents}
+import scala.io.Source
 
 class ManagementController (override val controllerComponents: ControllerComponents) extends BaseController with Logging {
   def manifest: Action[AnyContent] = Action {
     Ok(Json.parse(BuildInfo.toJson))
   }
 
+  def tags: Action[AnyContent] = Action {
+    /*
+    This file is created by https://github.com/guardian/instance-tag-discovery,
+    which is part of the `cdk-base` AMIgo role.
+     */
+    val filename = "/etc/config/tags.json"
+    try {
+      val content = Source.fromFile(filename)
+      val tagsJson = try content.mkString finally content.close()
+      Ok(Json.parse(tagsJson))
+    }
+    catch {
+      case e: Exception => InternalServerError(e.getMessage)
+    }
+  }
+
   def disallowRobots: Action[AnyContent] = Action {
     Ok("User-agent: *\nDisallow: /\n")
   }

conf/routes

@@ -1,4 +1,5 @@
 GET     /                           controllers.ManagementController.manifest
+GET     /tags                       controllers.ManagementController.tags
 GET     /healthcheck                controllers.ManagementController.healthCheck
 GET     /moved-permanently          controllers.ManagementController.movedPermanently
 GET     /bad-request                controllers.ManagementController.badRequest