Support for the method `Span.addTags`

[bedorlan]

The method Span.addTags is not supported.

Its documented here:
https://opentracing-javascript.surge.sh/classes/span.html#addtags

[DanielMSchmidt]

AFAIK Zipkin only provides one interface for span annotations, we need to use that I would guess. We currently implement log, so we could write it like this:

addTags(tags) {
	this.log(tags);
}

How would you feel about doing this change?