chore: Add CLIENT ID command (#3672)

We already adhere to all requirements, we just need to return the id
when the command is issued :)

Fixes #3651

src/server/server_family.cc

@@ -541,6 +541,14 @@ void ClientCaching(CmdArgList args, ConnectionContext* cntx) {
   cntx->SendOk();
 }
 
+void ClientId(CmdArgList args, ConnectionContext* cntx) {
+  if (args.size() != 0) {
+    return cntx->SendError(kSyntaxErr);
+  }
+
+  return cntx->SendLong(cntx->conn()->GetClientId());
+}
+
 void ClientKill(CmdArgList args, absl::Span<facade::Listener*> listeners, ConnectionContext* cntx) {
   std::function<bool(facade::Connection * conn)> evaluator;
 
@@ -1762,6 +1770,8 @@ void ServerFamily::Client(CmdArgList args, ConnectionContext* cntx) {
     return ClientKill(sub_args, absl::MakeSpan(listeners_), cntx);
   } else if (sub_cmd == "CACHING") {
     return ClientCaching(sub_args, cntx);
+  } else if (sub_cmd == "ID") {
+    return ClientId(sub_args, cntx);
   }
 
   if (sub_cmd == "SETINFO") {