@Nullable
    protected final BiFunction<String, HttpFileAttributes, String> entityTagFunction() {
        return entityTagFunction;
    }
Returns the function which generates the entity tag that's used for setting the {@code "etag"} header
automatically.

@return the etag function, or {@code null} if the {@code "etag"} header is not set automatically.
core/src/main/java/com/linecorp/armeria/server/file/AbstractHttpFileBuilder.java