title: Find substring in json's value and return the field
body: <p>I've got a nested json and I would like to find a substring in any pair's value. The results should be the pair's name or None or similar if not found at all. So for example let's assume that I am looking for the substring <code>&quot;met&quot;</code>. Then for:</p>
  <pre><code>{
    &quot;a&quot;: &quot;example&quot;,
    {
       &quot;b&quot;: &quot;another example&quot;,
       &quot;c&quot;: &quot;something else&quot;
    }
  }
  </code></pre>
  <p>result should be <code>&quot;c&quot;</code> (as <code>&quot;met&quot;</code> is found in <code>&quot;something else&quot;</code>)
  and for:</p>
  <pre><code>{
   &quot;a&quot;: &quot;example&quot;,
   {
      &quot;b&quot;: &quot;another example&quot;,
      &quot;c&quot;: &quot;yet another one&quot;
   }
  }
  </code></pre>
  <p>the result should be <code>None</code> as no <code>met</code> is found.</p>
  <p>I have no additional information about json.</p>
  <p>How to do it in the most efficient way?</p>