title: How to access (get) camel header in java dsl
body: <p>I have a code like - </p>
  
  <pre><code>        // use streaming to increase index throughput
              .setHeader(SolrConstants.OPERATION,
                      constant(SolrConstants.OPERATION_INSERT_STREAMING))
              // define solr endpoint and options
              .to("solr://"
                      + getSolrEndPoint()
                      + "?defaultMaxConnectionsPerHost=500&amp;streamingThreadCount=500&amp;maxRetries=3")
              .log(LoggingLevel.INFO, "Successfully indexed document id [" +header(BatchHeaders.DOCUMENT_ID) +"]")
              // end this route
              .end();
  </code></pre>
  
  <p>But what I m getting in the log is - </p>
  
  <pre><code>severity="INFO " thread="Camel (camel-1) thread #123 - seda://insert" category="route2" Successfully indexed document id [header{DOC_ID}]
  </code></pre>
  
  <p>I m not getting the actual header value (the document id).<br>
  So my question is - How to access the headers in Java DSL here?</p>