Metric expressjs_number_of_open_connections does not include prefix

```
const prometheusMiddleware = require('prometheus-api-metrics');
app.use(prometheusMiddleware({
  metricsPrefix: 'prefix',
}));
```

`expressjs_number_of_open_connections` should be `prefix_expressjs_number_of_open_connections` but it appears to be the only metric without the prefix