BUG/MINOR: contrib/prometheus-exporter: Use HTX errors and not legacy ones

This bug was introduced when the commit 32d634f1 ("MINOR:
contrib/prometheus-exporter: filter exported metrics by scope") was backported
to 2.0.

In 2.0, Error chunks exist in raw format (http_err_chunks[]) and in HTX format
(htx_err_chunks[]). Prometheus exported only supports the HTX. So error must not
be reported using the raw chunks.

This fix is specific to 2.0. No backport needed.
1 file changed