title: prometheus sum one NaN value result into NaN. How to avoid it?
body: <p>I have the following metrics in prometheus: it counts memcached command by sec:</p>
  
  <pre><code>sum (irate (memcached_commands_total{instance="memcached-instance"}[5m])) by (command)
  </code></pre>
  
  <p>Result:</p>
  
  <pre><code>{command="delete"}  0
  {command="flush"}   0
  {command="get"} 62.733333333333334
  {command="incr"}    0
  {command="set"} 93.43333333333334
  {command="touch"}   NaN
  {command="cas"} 0
  {command="decr"}    0
  </code></pre>
  
  <p>I want to count commands by sec (without separate rate for different commands). I have tried the following formula:</p>
  
  <pre><code>sum (irate (memcached_commands_total{instance="memcached-instance"}[5m]))
  </code></pre>
  
  <p>But the result is:</p>
  
  <pre><code>{}  NaN
  </code></pre>
  
  <p>I expect about 155, but it is NaN. I suppose it is command="touch" the culprit. It is possible to exclude NaN from the sum?</p>