lib: add unsubscribe method to non-active DC channels

PR-URL: #40433
Reviewed-By: Vladimir de Turckheim <vlad2t@hotmail.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Bryan English <bryan@bryanenglish.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>

doc/api/diagnostics_channel.md

@@ -267,7 +267,7 @@ added:
 changes:
   - version: REPLACEME
     pr-url: https://github.com/nodejs/node/pull/40433
-    description: Added return value.
+    description: Added return value. Added to channels without subscribers.
 -->
 
 * `onMessage` {Function} The previous subscribed handler to remove

lib/diagnostics_channel.js

@@ -81,6 +81,10 @@ class Channel {
     this.subscribe(subscription);
   }
 
+  unsubscribe() {
+    return false;
+  }
+
   get hasSubscribers() {
     return false;
   }