title: Create a custom Tensorflow histogram summary
body: <p>There are a couple of SO answers on creating a custom scalar summary in TF (<a href="https://stackoverflow.com/questions/37902705/how-to-manually-create-a-tf-summary">here</a> and <a href="https://stackoverflow.com/questions/37530228/how-do-i-add-an-arbitrary-value-to-a-tensorflow-summary">here</a>), but I can't find anything on creating a custom <em>histogram</em> summary. The documentation seems to be very lacking for custom summaries. I have a numpy array of that I'd like to make a summary of - any ideas on how?</p>
  
  <p>(tf.Summary.Value has a histo field that I tried using, but it requires a tensorflow::HistogramProto; there's no documentation on that class either, so I'm at a loss on how to make it. I've tried creating a minimal failing example below).</p>
  
  <pre><code>import tensorflow as tf
  import numpy as np
  sess = tf.Session()
  means_placeholder = tf.placeholder(tf.float32)
  tf.summary.histogram('means', means_placeholder)
  summaries = tf.summary.merge_all()
  writer = tf.summary.FileWriter('./summaries')
  means = np.random.random(10)    
  writer.add_summary(tf.Summary(value=[tf.Summary.Value(tag='means', histo=means)]))
  </code></pre>