title: How do you use blacklist_values an match all fields in the logstash-filter-prune plugin?
body: <p>My log messages contain a bunch of key/value pairs. I use <code>kv</code> to specify a list of keys to keep. However, I need to exclude keys which do not have a value. For this, I am using <code>gsub</code> to change an empty value into <code>notset</code>. Now I want to use <code>prune</code> to blacklist these values. I wasn't aware that you need to specify each field name individually with <code>prune</code>. Is there a way to match all fields? If I add new fields in the <code>kv</code> section, I do not want to touch the <code>blacklist_values</code> hash again. </p>
  
  <p>Here is an example Logstash filter configuration:</p>
  
  <pre><code>filter{
    mutate {
      gsub =&gt; [ 'message', '= ', '=notset ' ]
    }
    kv {
      include_keys =&gt; ["a", "b", "c", "d", "e" ]
    }
    prune {
      blacklist_values =&gt; [ "??", "notset" ]
    }
    ...
  }
  </code></pre>