"Application metrics" not being exported after setting ENABLE_METRICS

Chart version: 3.6.2

We've set ENABLE_METRICS:

```
/ $ strings /proc/1/environ | grep -i metrics
ENABLE_METRICS=true
```

...however, the metrics `chartmuseum_charts_served_total` and `chartmuseum_chart_versions_served_total` documented [here](https://github.com/helm/chartmuseum#prometheus-metrics) aren't being exported.

```
❯ curl -s localhost:8080/metrics | grep -v "#" | grep chartmuseum_chart | wc -l
0
```

The "general global metrics" are being exported:

```
❯ curl -s localhost:8080/metrics | grep -v "#" | grep -i chartmuseum
chartmuseum_request_duration_seconds_sum 11.347815388000111
chartmuseum_request_duration_seconds_count 330745
chartmuseum_request_size_bytes_sum 2.8782004e+07
chartmuseum_request_size_bytes_count 330745
chartmuseum_requests_total{code="200",handler="helm.sh/chartmuseum/pkg/chartmuseum/router.(*Router).rootHandler-fm",host="x:8080",method="GET",url="/health"} 330192
chartmuseum_requests_total{code="200",handler="helm.sh/chartmuseum/pkg/chartmuseum/router.(*Router).rootHandler-fm",host="x:8080",method="GET",url="/:repoindex.yaml"} 553
chartmuseum_response_size_bytes_sum 4.34963519e+08
chartmuseum_response_size_bytes_count 330745
```

Reproduce the issue with ChartMuseum 3.6.2 and ENABLE_METRICS=true, then query localhost:8080/metrics and compare the output with the documented chartmuseum_charts_served_total and chartmuseum_chart_versions_served_total metrics. Done means those application metrics are exported alongside the existing general global metrics.

Written by the indexing model from the issue text.