chore: Strict bounds when adding uknown command for INFO ALL stats (#5735)

Currently we allow 1024 uknown command with max length of 4096
characters to be stored in temporary vector. Create more strict
rule that allows only 64 entries with max length of 20 chars.

Closes #5732

Author: mkaruza

src/server/main_service.cc

@@ -1805,8 +1805,11 @@ void Service::DispatchMC(const MemcacheParser::Command& cmd, std::string_view va
 }
 
 ErrorReply Service::ReportUnknownCmd(string_view cmd_name) {
+  constexpr uint8_t kMaxUknownCommands = 64;
+  constexpr uint8_t kMaxUknownCommandLength = 20;
+
   lock_guard lk(mu_);
-  if (unknown_cmds_.size() < 1024)
+  if (unknown_cmds_.size() <= kMaxUknownCommands && cmd_name.size() <= kMaxUknownCommandLength)
     unknown_cmds_[cmd_name]++;
 
   return ErrorReply{StrCat("unknown command `", cmd_name, "`"), "unknown_cmd"};