Metric comments generated even when no metrics

Following the guidance here: https://docs.fluentd.org/monitoring-fluentd/monitoring-prometheus#step-1-counting-incoming-records-by-prometheus-filter-plugin

Setting up an output plugin metric also generates `HELP` and `TYPE` comments for other possible but not provided metrics.

Whilst the spec technically allows this (the wording is they can only exist a maximum of once) it can confuse scraping tools and does not seem right: we should only generate those special comments when a metric exists.

An example using this config:
```

@type forward
port 5000

@type prometheus
bind 0.0.0.0
port 24231
metrics_path /metrics

@type prometheus_output_monitor
interval 10

hostname ${hostname}

@type null

```
This when run produces empty metrics for some which then some scrapers do not like:
```shell
$ docker run --user 0 --rm -it -p 24231:24231 -v $PWD:/fluentd/etc ghcr.io/calyptia/fluentd:edge-debian sh -c "fluent-gem install fluent-plugin-prometheus; su fluentd; tini -- /bin/entrypoint.sh -c /fluentd/etc/fluentd.conf"
...
$ curl localhost:24231/metrics
# TYPE fluentd_output_status_buffer_total_bytes gauge
# HELP fluentd_output_status_buffer_total_bytes Current total size of stage and queue buffers.
# TYPE fluentd_output_status_buffer_stage_length gauge
# HELP fluentd_output_status_buffer_stage_length Current length of stage buffers.
# TYPE fluentd_output_status_buffer_stage_byte_size gauge
# HELP fluentd_output_status_buffer_stage_byte_size Current total size of stage buffers.
# TYPE fluentd_output_status_buffer_queue_length gauge
# HELP fluentd_output_status_buffer_queue_length Current length of queue buffers.
# TYPE fluentd_output_status_queue_byte_size gauge
# HELP fluentd_output_status_queue_byte_size Current total size of queue buffers.
# TYPE fluentd_output_status_buffer_available_space_ratio gauge
# HELP fluentd_output_status_buffer_available_space_ratio Ratio of available space in buffer.
# TYPE fluentd_output_status_buffer_newest_timekey gauge
# HELP fluentd_output_status_buffer_newest_timekey Newest timekey in buffer.
# TYPE fluentd_output_status_buffer_oldest_timekey gauge
# HELP fluentd_output_status_buffer_oldest_timekey Oldest timekey in buffer.
# TYPE fluentd_output_status_retry_count gauge
# HELP fluentd_output_status_retry_count Current retry counts.
fluentd_output_status_retry_count{hostname="df5808e5adb2",plugin_id="object:834",type="null"} 0.0
# TYPE fluentd_output_status_num_errors gauge
# HELP fluentd_output_status_num_errors Current number of errors.
fluentd_output_status_num_errors{hostname="df5808e5adb2",plugin_id="object:834",type="null"} 0.0
# TYPE fluentd_output_status_emit_count gauge
# HELP fluentd_output_status_emit_count Current emit counts.
fluentd_output_status_emit_count{hostname="df5808e5adb2",plugin_id="object:834",type="null"} 3.0
# TYPE fluentd_output_status_emit_records gauge
# HELP fluentd_output_status_emit_records Current emit records.
fluentd_output_status_emit_records{hostname="df5808e5adb2",plugin_id="object:834",type="null"} 3.0
# TYPE fluentd_output_status_write_count gauge
# HELP fluentd_output_status_write_count Current write counts.
fluentd_output_status_write_count{hostname="df5808e5adb2",plugin_id="object:834",type="null"} 0.0
# TYPE fluentd_output_status_rollback_count gauge
# HELP fluentd_output_status_rollback_count Current rollback counts.
fluentd_output_status_rollback_count{hostname="df5808e5adb2",plugin_id="object:834",type="null"} 0.0
# TYPE fluentd_output_status_flush_time_count gauge
# HELP fluentd_output_status_flush_time_count Total flush time.
fluentd_output_status_flush_time_count{hostname="df5808e5adb2",plugin_id="object:834",type="null"} 0.0
# TYPE fluentd_output_status_slow_flush_count gauge
# HELP fluentd_output_status_slow_flush_count Current slow flush counts.
fluentd_output_status_slow_flush_count{hostname="df5808e5adb2",plugin_id="object:834",type="null"} 0.0
# TYPE fluentd_output_status_retry_wait gauge
# HELP fluentd_output_status_retry_wait Current retry wait
fluentd_output_status_retry_wait{hostname="df5808e5adb2",plugin_id="object:834",type="null"} 0.0
```