title: Boost logger append to file
body: <p>I have initialized sink that logs to file:</p>
  
  <pre><code>logging::add_file_log
              (
              keywords::file_name = "sample_%N.log",                                        /*&lt; file name pattern &gt;*/
              keywords::rotation_size = 10 * 1024 * 1024,                                   /*&lt; rotate files every 10 MiB... &gt;*/
              keywords::time_based_rotation = sinks::file::rotation_at_time_point(0, 0, 0), /*&lt; ...or at midnight &gt;*/
              keywords::format = "[%TimeStamp%]: %Message%",                                 /*&lt; log record format &gt;*/
              keywords::auto_flush = true
              );
  </code></pre>
  
  <p>It looks it rewrites file during program restart. How to make it append to file?</p>