num_errors metric missing with fluentd 1.14

[stevenjm]

Since the metrics changes in fluentd 1.14 (https://github.com/fluent/fluent-plugin-prometheus/pull/191), the fluentd_output_status_num_errors metric is missing. To reproduce:

1. Clone the https://github.com/fluent/fluentd repo.
2. git checkout v1.14.0
3. Gemfile.local

   gem "fluent-plugin-prometheus", "2.0.2"

4. fluent.conf

   
   <source>
   @type tail
   @id in
   path in
   tag test
   read_from_head true
   <parse>
   @type none
   </parse>
   </source>

<match test.**> @type file @id out path out

@type prometheus

@type prometheus_output_monitor

5. `bundle install`
6. `bundle exec fluentd -c fluent.conf`
7. There is no `num_errors` metric...

$ curl -s http://127.0.0.1:24231/metrics | grep num_errors

TYPE fluentd_output_status_num_errors gauge

HELP fluentd_output_status_num_errors Current number of errors.

8. ... but the same metric is available under the old `retry_count` name.

$ curl -s http://127.0.0.1:24231/metrics | grep retry_count

TYPE fluentd_output_status_retry_count gauge

HELP fluentd_output_status_retry_count Current retry counts.

fluentd_output_status_retry_count{plugin_id="out",type="file"} 0.0

To confirm it works with previous versions:

1. `git checkout v1.13.3`
2. `bundle exec fluentd -c fluent.conf`
3.

$ curl -s http://127.0.0.1:24231/metrics | grep num_errors

TYPE fluentd_output_status_num_errors gauge

HELP fluentd_output_status_num_errors Current number of errors.

fluentd_output_status_num_errors{plugin_id="out",type="file"} 0.0