num_errors metric missing with fluentd 1.14

[stevenjm]

Since the metrics changes in fluentd 1.14 (#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
</match>

<source>
@type prometheus
</source>

<source>
@type prometheus_output_monitor
</source>

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

$ 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