Metric library only accepts array for the namespace.

When you use the metric library directly, the first arguments of any methods like the **gauge** method need to be an explicit array. The metric library is mostly only used in the periodic poller or other internal classes it never face plugin developer.

```ruby
metric.gauge(:jvm, :uptime_in_millis, 123)
service.get_shallow(:jvm, :uptime_in_millis) # fails

metric.gauge([:jvm], :uptime_in_millis, 123)
service.get_shallow(:jvm, :uptime_in_millis) # succeed
```

Code: https://github.com/elastic/logstash/blob/fix/6214/logstash-core/lib/logstash/instrument/metric.rb

This issue was discovered by @suyograo