title: Boost Log rotation filename pattern integer never restarted
body: <p>I'm trying to make a logging system that stores the three previous logs in a directory "stored" and the current one. I'm using the pattern filename_%2N. Everything is working fine, but testing it, the filename integer is never restarted, so after some time, I'm getting files like log_2020.log.  Even if I set fileConfig.numFiles to 3, the number goes to N. When this integer is restarted? Is there any way to do it? </p>
  
  <pre class="lang-cpp prettyprint-override"><code>boost::shared_ptr&lt;fileSink_t&gt; file(new FileAppender::fileSink_t(
       boost::log::keywords::open_mode = (std::ios::app | std::ios::out),
       boost::log::keywords::file_name = fileConfig.path+fileConfig.filename+"_%2N.log",
       boost::log::keywords::rotation_size = fileConfig.maxSizekb * 1024,
       boost::log::keywords::auto_flush = true
  ));
  file-&gt;locked_backend()-&gt;set_file_collector(boost::log::sinks::file::make_collector(
      boost::log::keywords::target = fileConfig.path+"/stored",
      boost::log::keywords::max_files = fileConfig.numFiles
  ));
  file-&gt;locked_backend()-&gt;scan_for_files();
  </code></pre>