@Override
    public <REQUEST, RESPONSE> ServerCall.Listener<REQUEST> interceptCall(ServerCall<REQUEST, RESPONSE> serverCall,
        Metadata metadata, ServerCallHandler<REQUEST, RESPONSE> serverCallHandler) {
        String token = metadata.get(AUTH_HEAD_HEADER_NAME);
        if (expectedToken.equals(token)) {
            return serverCallHandler.startCall(serverCall, metadata);
        } else {
            serverCall.close(Status.PERMISSION_DENIED, new Metadata());
            return listener;
        }
    }
intercept point of call.

@param serverCall        call of server.
@param metadata          of call.
@param serverCallHandler handler of call.
@param <REQUEST>         of call.
@param <RESPONSE>        of call.
@return lister of call.
oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/server/auth/AuthenticationInterceptor.java