code: public static CallCredentials bearerAuth(final String token) {
          final Metadata extraHeaders = new Metadata();
          extraHeaders.put(AUTHORIZATION_HEADER, BEARER_AUTH_PREFIX + token);
          return new StaticSecurityHeaderCallCredentials(extraHeaders);
      }
docs: Creates a new call credential with the given token for bearer auth.
  
  <p>
  <b>Note:</b> This method uses experimental grpc-java-API features.
  </p>
  
  @param token the bearer token to use
  @return The newly created bearer auth credentials.