improve enable_metric docs (was: enable_metric => false in plugin configuration does nothing)

- Version: 5.4.1
- Operating System: Linux
- Config File (if you have sensitive info, please remove it):

(any tested input/filter/outplut plugin behaves the same way):
```
input { beats { enable_metric => false } }
```

- Sample Data: / Steps to Reproduce:

Followup from https://discuss.elastic.co/t/input-filter-output-metrics-still-exposed-after-setting-enable-metric-false/89088

After restarting logstash STATS API will still provide metrics for all plugins, including the ones with `enable_metric => false`

`curl -s 'http://127.0.0.1:9600/_node/stats?pretty'`

```
[...]
"plugins" : {
"inputs" : [ {
"id" : "redacted-1",
"events" : {
"out" : 1665108,
"queue_push_duration_in_millis" : 332750
},
"name" : "beats"
} ],
[...]
```