static AccessLogWriter custom(String formatStr) {
        requireNonNull(formatStr, "formatStr");
        final List<AccessLogComponent> accessLogFormat = parseCustom(formatStr);
        checkArgument(!accessLogFormat.isEmpty(), "Invalid access log format string: %s", formatStr);
        return requestLog -> AccessLogger.write(accessLogFormat, requestLog);
    }
Returns an access log writer with the specified {@code formatStr}.
core/src/main/java/com/linecorp/armeria/server/logging/AccessLogWriter.java