title: how to add custom field to log logback-logstash
body: <p>My spring boot log currently looks like the following.</p>
  
  <pre><code>{"@timestamp":"2018-08-07T14:49:21.244+01:00","@version":"1","message":"Starting Application on ipkiss bla bla)","logger_name":"logger name....","thread_name":"main","level":"INFO","level_value":20000}
  </code></pre>
  
  <p>with the logback-spring.xml setup like below</p>
  
  <pre><code>&lt;appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender"&gt;
          &lt;encoder class="com.ipkiss.correlate.logback.CorrelationPatternLayoutEncoder"&gt;
              &lt;pattern&gt;%clr(%d{yyyy-MM-dd HH:mm:ss.SSS}){faint} %clr(%5p) %clr(${PID:- }){magenta} %clr(---){faint} %clr([%15.15t]){faint} %clr(%-40.40logger{39}){cyan} %clr(:){faint} id = %id  %m%n%wEx&lt;/pattern&gt;
          &lt;/encoder&gt;
  &lt;/appender&gt;
  </code></pre>
  
  <p>and my class for LayoutEncoder looks like this</p>
  
  <pre><code>public class CorrelationPatternLayoutEncoder extends PatternLayoutEncoder {
      public CorrelationPatternLayoutEncoder() {
  
      }
  
  @Override
  public void start() {
      PatternLayout patternLayout = new PatternLayout();
      patternLayout.getDefaultConverterMap().put("id", CorrelationConverter.class.getName());
      patternLayout.setContext(context);
      patternLayout.setPattern(getPattern());
      patternLayout.setOutputPatternAsHeader(outputPatternAsHeader);
      patternLayout.start();
      this.layout = patternLayout;
      this.started = true;
  }
  </code></pre>
  
  <p>}</p>
  
  <p>what I was trying to achieve is to add the id to the log, I can't make logstach append my id, i tried Custom field according to the docs but I couldn't make it work, 
  any ideas how I can achieve this? </p>
  
  <p>this is what i want to end up with </p>
  
  <pre><code>{"id":"3a7ccd34-d66a-4fcc-a12e-763a395a496c","@timestamp":"2018-08-07T14:49:21.244+01:00","@version":"1","message":"Starting Application on ipkiss bla bla)","logger_name":"logger name....","thread_name":"main","level":"INFO","level_value":20000}
  </code></pre>
  
  <p>or id being appended at the end of the log.</p>