Add the possiblity to define a callback to set a key

src/stream/stream-producer-preparer.ts

 import { Transform } from 'stream';
+import {Key} from 'readline';
 
 /**
  * A type to for the PartitionCallback function
  */
 
 export type PartitionCallback = (chunk: any) => number;
+export type KeyCallback = (chunk: any) => number;
 /**
  * Prepare a value for kafka ProducerStream
  */
@@ -20,10 +22,16 @@ class StreamProducerPreparer extends Transform {
    */
   private readonly partition: number |  PartitionCallback = -1;
 
+  /**
+   * Should we use a key or not
+   */
+  private readonly key: boolean | KeyCallback = false;
+
   constructor(opts) {
     super(opts);
     this.topic = opts.topic;
     this.partition = opts.partition || -1;
+    this.key = opts.key || false;
   }
   /**
    * @inheritDoc
@@ -34,11 +42,14 @@ class StreamProducerPreparer extends Transform {
    */
   public _transform(chunk, encoding, callback) {
     // see: producer-stream.js L: 176
-    const value = {
+    const value: any  = {
       topic: this.topic,
       value: chunk,
       partition: typeof this.partition === 'function' ? this.partition(chunk) : this.partition,
     };
+    if (typeof this.key === 'function') {
+      value.key = this.key(chunk);
+    }
     callback(null, value);
   }
 }