setAttribute value should be required

[dyladan]

The span.setAttribute() API could be improved. It tells null and undefined are not allowed for value but type definition has value optional.

opentelemetry-js/packages/opentelemetry-api/src/trace/span.ts

Lines 44 to 53 in 1a3f3f8

	/**
	* Sets an attribute to the span.
	*
	* Sets a single Attribute with the key and value passed as arguments.
	*
	* @param key the key for this attribute.
	* @param value the value for this attribute. Setting a value null or
	* undefined is invalid and will result in undefined behavior.
	*/
	setAttribute(key: string, value?: AttributeValue): this;

Originally posted by @Flarna in #1765 (comment)

[dyladan]

null and undefined is officially undefined behavior according to the spec. We should change the typings to reflect that it is not allowed.