An error has occurred while serving routing_http_client metrics

### Version

```Text
0.20.0
```

### Config

```
"Routing": {
"Methods": null,
"Routers": null,
"Type": "auto"
},
```

### Description

http://127.0.0.1:5001/debug/metrics/prometheus ends up in broken state after a multiple days of uptime:

```
An error has occurred while serving metrics:

14 error(s) occurred:
* collected metric "routing_http_client_length" { label: label: histogram: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: > } was collected before with the same name and label values
* collected metric "routing_http_client_latency" { label: label: label: label: histogram: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: > } was collected before with the same name and label values
* collected metric "routing_http_client_latency" { label: label: label: label: histogram: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: > } was collected before with the same name and label values
* collected metric "routing_http_client_latency" { label: label: label: label: histogram: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: > } was collected before with the same name and label values
* collected metric "routing_http_client_latency" { label: label: label: label: histogram: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: > } was collected before with the same name and label values
* collected metric "routing_http_client_latency" { label: label: label: label: histogram: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: > } was collected before with the same name and label values
* collected metric "routing_http_client_latency" { label: label: label: label: histogram: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: > } was collected before with the same name and label values
* collected metric "routing_http_client_latency" { label: label: label: label: histogram: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: > } was collected before with the same name and label values
* collected metric "routing_http_client_latency" { label: label: label: label: histogram: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: > } was collected before with the same name and label values
* collected metric "routing_http_client_latency" { label: label: label: label: histogram: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: > } was collected before with the same name and label values
* collected metric "routing_http_client_latency" { label: label: label: label: histogram: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: > } was collected before with the same name and label values
* collected metric "routing_http_client_latency" { label: label: label: label: histogram: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: > } was collected before with the same name and label values
* collected metric "routing_http_client_latency" { label: label: label: label: histogram: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: > } was collected before with the same name and label values
* collected metric "routing_http_client_length" { label: label: histogram: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: > } was collected before with the same name and label values
```

Seems we have a bug in `boxo/routing/http/client`, needs analysis.