docs: public static Set<Attribute> context(String key, String value){
          if(null==key) {
              throw new IllegalArgumentException("key cannot be null");
          }
          if(null==value){
              throw new IllegalArgumentException("value cannot be null");
          }
          return Collections.singleton(
                  new Attribute(
                          URI.create(EnvironmentalContext.URI_BASE + key),
                          value
                  )
          );
      }