title: ScalaPB code generation option for metadata access in method implementation
body: <p>What option can I set for ScalaPB code generation where the methods for the service implementation have access to metadata?</p>
  <p>Default options generate something like:</p>
  <pre class="lang-scala prettyprint-override"><code>override def sayHello(req: HelloRequest) = {
  ...
  }
  </code></pre>
  <p>And I would like something like:</p>
  <pre class="lang-scala prettyprint-override"><code>override def sayHello(req: HelloRequest, metadata: Metadata) = {
  ...
  }
  </code></pre>
  <p>--- Update December, 30th, 2021 ---</p>
  <p>One reason is that I might want to retrieve the client's token from the metadata, so I can use that token to call another service.</p>