title: multi-file backend and severity
body: <p>I configured boost log to have multiple log files but i have problem with severity level. I can't print this in log.</p>
  
  <pre><code>enum severity_level
  {
      error = 0,
      critical = 1,
      info = 2,
      warning = 3,
      debug = 4,
      trace = 5
  };
  
  BOOST_LOG_INLINE_GLOBAL_LOGGER_DEFAULT(my_logger, boost::log::sources::logger_mt);
  BOOST_LOG_ATTRIBUTE_KEYWORD(severity, "Severity", severity_level);
  </code></pre>
  
  <p>.....</p>
  
  <pre><code>std::ostream&amp; operator&lt;&lt; (std::ostream&amp; strm, severity_level level)
  {
      static const char* severity_name[] = {
          "ERR",
          "CRI",
          "INF",
          "WAR",
          "DEB",
          "TRA"
      };
  
      if (static_cast&lt; std::size_t &gt;(level) &lt; sizeof(severity_name) / sizeof(*severity_name))
          strm &lt;&lt; severity_name[level];
      else
          strm &lt;&lt; static_cast&lt; int &gt;(level);
  
      return strm;
  }
  
  void log_init()
  {
      // Create a text file sink
      typedef boost::log::sinks::synchronous_sink&lt; boost::log::sinks::text_multifile_backend &gt; file_sink;
      boost::shared_ptr&lt; file_sink &gt; sink(new file_sink);
  
      // Set up how the file names will be generated
      sink-&gt;locked_backend()-&gt;set_file_name_composer(boost::log::sinks::file::as_file_name_composer(
      boost::log::expressions::stream &lt;&lt; "logs/" &lt;&lt; boost::log::expressions::attr&lt; std::string &gt;("logger_name") &lt;&lt; ".log"));
  
      // Set the log record formatter
      sink-&gt;set_formatter
      (
      boost::log::expressions::format("%1%: [%2%] - %3%")
      % boost::log::expressions::attr&lt;severity_level&gt;("Severity")
      % boost::log::expressions::attr&lt; boost::posix_time::ptime &gt;("TimeStamp")
      % boost::log::expressions::smessage
      );
  
      // Add it to the core
      boost::log::core::get()-&gt;add_sink(sink);
  
      // Add some attributes
      boost::log::core::get()-&gt;add_global_attribute("TimeStamp", boost::log::attributes::local_clock());
      boost::log::core::get()-&gt;add_global_attribute("RecordID", boost::log::attributes::counter&lt; unsigned int &gt;());
  }
  </code></pre>
  
  <p>But when i try to print log i got no severity in log file.</p>
  
  <pre><code>BOOST_LOG_SEV(my_logger::get(), severity_level::info)&lt;&lt;"some text logged";
  : [2013-Aug-13 11:07:18.280748] - some text logged
  </code></pre>
  
  <p>What i'm doing wrong?</p>