title: How to log MDC with Spring Sleuth?
body: <p>I have a Spring boot + sleuth based application. All works as expected. I have for now logs like this:</p>
  
  <pre><code>2017-05-04 17:55:52.226  INFO [alert,692d0eeca479e216,c3c8b680dc29ad02,false] 17292 --- [cTaskExecutor-1] c.k.a.b.s.alert.impl.AlertServiceImpl    : Alert state to process: xxx
  </code></pre>
  
  <p>Now, I want to add custom MDC to my log like the contract reference for example. I want to have logs like this:</p>
  
  <pre><code>2017-05-04 17:55:52.226  INFO [alert,692d0eeca479e216,c3c8b680dc29ad02,false] [CONTRACT_REF] 17292 --- [cTaskExecutor-1] c.k.a.b.s.alert.impl.AlertServiceImpl    : Alert state to process: xxx
  </code></pre>
  
  <p>I tried various things with no success:</p>
  
  <ol>
  <li>Use the Spring Sleuth Tracer to add a tag;</li>
  <li>Add logging.pattern.level=%5p %mdc to my application.properties file with MDC.put(xxx, xxx)</li>
  </ol>
  
  <p>How can I add custom MDC/tags to my log?</p>