code: @Nonnull
      public ZipkinAnnotationsStore addAnnotation(long timestamp, @Nonnull String s) {
          return addAnnotation(timestamp, s, defaultEndpoint);
      }
docs: Adds an annotation for an event that happened on a specific timestamp.
  
  @param timestamp The timestamp of the annotation, in microseconds
  @param s         The annotation value to emit
  @return this object