assign to undeclared variable 'str' in strict mode

[RepentantGopher]

Hi, i got error when tried to get access to metrics via http.

Unhandled error: /usr/local/share/lua/5.1/tarantool-prometheus.lua:322: assign to undeclared variable 'str' stack traceback: /usr/local/share/lua/5.1/http/server.lua:675: in function 'process_client' /usr/local/share/lua/5.1/http/server.lua:1113: in function </usr/local/share/lua/5.1/http/server.lua:1113> [C]: in function 'pcall' builtin/socket.lua:984: in function <builtin/socket.lua:982>

Lua was running in strict mode. When i turned off strict mode everything works fine.
I think root of this error is here https://github.com/tarantool/prometheus/blob/master/tarantool-prometheus.lua#L321