title: How to make Keras compute a certain metric on validation data only?
body: <p>I'm using <code>tf.keras</code> with TensorFlow 1.14.0. I have implemented a custom metric that is quite computationally intensive and it slows down the training process if I simply add it to the list of metrics provided as <code>model.compile(..., metrics=[...])</code>.</p>
  
  <p>How do I make Keras skip computation of the metric during training iterations but compute it on validation data (and print it) at the end of each epoch?</p>