Make Ladon initialization thread-safe

[lucarin91]

Do you want to request a feature or report a bug?

This is probably a feature request

What is the current behavior?

AFAIK the Ladon struct could be easily considered thread-safe because all its internal fields (e.g., Matcher, Manager) handle concurrency internally. Although, the lazy initialization isn't thread-safe. For instance, the metric function has a data race because multiple callers can concurrently try to initialize the matcher to the DefaultMetric .

func (l *Ladon) metric() Metric {
	if l.Metric == nil {
		l.Metric = DefaultMetric
	}
	return l.Metric

If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem.

What is the expected behavior?

I didn't find any claim that the Ladon struct is thread-safe, but I think with some small changes we can improve this and make a clear statement that this is safe to use from multiple goroutines.

Here is a possible solution with sync.Once lucarin91#1 which will keep the same interface. But I think we should consider changing the interface and province a constructor, which will be better and simple

Which version of the software is affected?

I am testing on master 972387f