code: public static Exemplar create(
        double value, Timestamp timestamp, Map<String, AttachmentValue> attachments) {
      checkNotNull(attachments, "attachments");
      Map<String, AttachmentValue> attachmentsCopy =
          Collections.unmodifiableMap(new HashMap<String, AttachmentValue>(attachments));
      for (Entry<String, AttachmentValue> entry : attachmentsCopy.entrySet()) {
        checkNotNull(entry.getKey(), "key of attachments");
        checkNotNull(entry.getValue(), "value of attachments");
      }
      return new AutoValue_Exemplar(value, timestamp, attachmentsCopy);
    }
docs: Creates an {@link Exemplar}.
  
  @param value value of the {@link Exemplar} point.
  @param timestamp the time that this {@code Exemplar}'s value was recorded.
  @param attachments the contextual information about the example value.
  @return an {@code Exemplar}.
  @since 0.20