void updateCredentials(String username, String password) {
    String token = username + ':' + password;
    basicCredentials = "Basic " + Base64.getEncoder().encodeToString(token.getBytes(UTF_8));
  }
Generate Elasticsearch basic user credentials.

<p>Ref: <a href="https://www.elastic.co/guide/en/x-pack/current/how-security-works.html"> How
Elasticsearch security works</a></p>
zipkin-server/src/main/java/zipkin2/server/internal/elasticsearch/BasicCredentials.java