config: "metrics-dump"->"to-json" option removed.
json_dump: HANDY_JSON_DUMP() constructs text representation of HANDY_METRICS_DUMP() and returns std::string (instead of std::shared_ptr<const std::string>).
json_dump: HANDY_JSON_DUMP() always returns non-empty std::string as empty metrics dump is represented by "{}".
json: fill() and to_string() methods that construct rapidjson::Value and std::string representations of metrics dump are in handystats::json namespace (was handystats::json_dump).