public AbstractHttpFileBuilder entityTag(BiFunction<String, HttpFileAttributes, String> entityTagFunction) {
        this.entityTagFunction = requireNonNull(entityTagFunction, "entityTagFunction");
        return this;
    }
Sets the function which generates the entity tag that's used for setting the {@code "etag"} header
automatically.

@param entityTagFunction the entity tag function that generates the entity tag, or {@code null}
                         to disable setting the {@code "etag"} header.
core/src/main/java/com/linecorp/armeria/server/file/AbstractHttpFileBuilder.java