c/core/test: Some metrics do not include timestamps. Possibly a bug in the library?

[ian-abbott]

In the C test code "c/core/test/test.c", metrics added to the payload by add_simple_metric() have timestamps, but metrics added to the payload by init_metric() followed by add_metric_to_payload() do not have timestamps unless is_historical is true and is_transient is false. That is due to the following code in init_metric() in "tahu.c":

    if (is_historical && !is_transient) {
        metric->has_timestamp = true;
        metric->timestamp = get_current_timestamp();
    }

Is that code correct? I would have thought that the default would be to add a timestamp, but some code adding a historical metric would want to fill in the appropriate time themselves.