prom-client: add category for connection handler

packages/hydrooj/src/handler/home.ts

@@ -363,6 +363,8 @@ class HomeDomainCreateHandler extends Handler {
 }
 
 class HomeMessagesHandler extends Handler {
+    category = '#message';
+
     async get() {
         // TODO(iceboy): projection, pagination.
         const messages = await message.getByUser(this.user._id);

packages/hydrooj/src/handler/judge.ts

@@ -172,6 +172,7 @@ export class SubmissionDataDownloadHandler extends Handler {
 }
 
 class JudgeConnectionHandler extends ConnectionHandler {
+    category = '#judge';
     processing: any = null;
     closed = false;
     query: any = { type: 'judge' };