title: How to put mono to the current context in reactor
body: <p>I got a mono&lt;*&gt; from reactive repository, I want put its value to the context, but I did not found a correct api.</p>
  <p>kotlin code:</p>
  <pre><code>val user:Mono&lt;User&gt; = userRepository.findById(userId)
  </code></pre>
  <p>How do I put its value to the reactor context, like this:</p>
  <pre><code>userRepository.findById(userId).doOnNext {
      // got current context...
      context[&quot;USER&quot;] = it
  }
  </code></pre>