title: How to add a message property in node azure iot sdk
body: <p>With the azure C sdk it's possible to add properties to the message envelope using <code>Map_AddOrUpdate(propMap, "propKey", propText)</code>.  Is there something similar?  I'm using</p>
  
  <pre><code>const Protocol = require('azure-iot-device-mqtt').Mqtt;
  const Client = require('azure-iot-device').Client;
  const Message = require('azure-iot-device').Message;
  
  client = Client.fromConnectionString(deviceConnectionString, Protocol);
  // misc code
   return new Promise( (resolve,reject) =&gt; {
      client.sendEvent(message,(err,res) =&gt; {
        if ( err ) {
          logger.error(TAG,'Failed to send message: ', err.toString())
          reject(err)
        }
        if (res) logger.info(TAG,'Send status: ' + res.constructor.name);
        resolve(res)
      })
    })
  </code></pre>
  
  <p>I was unable to find anything in the client interface that allowed me to set envelope properties.</p>