@Override
        public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
            log.warn("Task {} rejected from {}", r.toString(), executor.toString());
        }
Require for `server.crt` and `server.pem` for open ssl at server side.

@param certChainFile  `server.crt` file
@param privateKeyFile `server.pem` file
oap-server/server-library/library-server/src/main/java/org/apache/skywalking/oap/server/library/server/grpc/GRPCServer.java