Functions' and fields' visibility should be the same with sentinel-golang

[flearc]

Compared to sentinel-golang, some functions and struct fields have different visibility in sentinel-rust.

For example, form_metric_filename in golang is public but in rust it is private.

    // Generate the metric file name from the service name.
    func FormMetricFileName(serviceName string, withPid bool) string {
	dot := "."
	separator := "-"
	if strings.Contains(serviceName, dot) {
		serviceName = strings.ReplaceAll(serviceName, dot, separator)
	}
	filename := serviceName + separator + MetricFileNameSuffix
	if withPid {
		pid := os.Getpid()
		filename = filename + ".pid" + strconv.Itoa(pid)
	}
	return filename
}

// Generate the metric file name from the service name.
fn form_metric_filename(service_name: &str, with_pid: bool) -> String {
    let separator = "-";
    let mut filename = if service_name.contains('.') {
        service_name.replace('.', separator)
    } else {
        service_name.to_string()
    };

    filename.push_str(&format!("{}{}", separator, METRIC_FILENAME_SUFFIX));

    if with_pid {
        let pid = std::process::id();
        filename.push_str(&format!(".pid{}", pid));
    }

    filename
}