Repository: traefik/traefik (language: go)

Issue:
Some metrics are missing in v3
### Welcome!

- [X] Yes, I've searched similar issues on [GitHub](https://github.com/traefik/traefik/issues) and didn't find any.
- [X] Yes, I've searched similar issues on the [Traefik community forum](https://community.traefik.io) and didn't find any.

### What did you do?

I'm using traefik v3.0.1 while debugging my application, I noticed that traefik is missing some metrics and it seems it started with v3.
I noticed that the following metrics are missing:

```
traefik_config_reloads_failure_total
traefik_config_last_reload_failure
traefik_entrypoint_open_connections
traefik_service_open_connections
traefik_service_retries_total
traefik_service_server_up
```

In particular, I was most interested in
```
traefik_service_open_connections
```

but these metrics are not in the versions **v3.0.0** , **v3.0.1** , **v3.1.0**

In version **v2.11.6** there is

### What did you see instead?

There is no part of the metrics in versions v3+

### What version of Traefik are you using?

- v3.0.0
- v3.0.1
- v3.1.0
- v2.11.6

### What is your environment & configuration?

Static:
```yaml
entryPoints:
  web:
    address: ":8081"

api:
  dashboard: true
  insecure: true

metrics:
  prometheus: {}

providers:
  file:
    directory: conf
    watch: true
```

Dynamic:
```yaml
http:
  services:
    test:
      loadBalancer:
        servers:
          - url: http://127.0.0.1:8080
  routers:
    test:
      rule: "PathPrefix(`/`)" 
      entryPoints:
        - "web"
      service: "test"
```


### If applicable, please paste the log output in DEBUG level

_No response_


Discussion / hints:
Hello @jidckii,

Metrics have been reworked in v3, and notably with https://github.com/traefik/traefik/pull/9656.

The equivalent to `traefik_service_open_connections` metrics is now a [global metric](https://doc.traefik.io/traefik/observability/metrics/overview/#global-metrics): `traefik_open_connections`.

We are keeping this issue as a request for a fix of our documentation in that regard in the migration guide.
Additionally to my previous comment: 

The `traefik_config_reloads_failure_total` and `traefik_config_last_reload_failure` metrics have been suppressed since they could not be implemented: https://github.com/traefik/traefik/pull/9660.

The `traefik_service_retries_total` metric, it has never been implemented as well: https://github.com/traefik/traefik/blob/6b1adabeb593446351b94966f6673058ab58564f/pkg/server/middleware/middlewares.go#L333
Regarding this one, I think we could stop documenting it, or fix it.

The `traefik_service_server_up` metric should work for service configured with healthcheck.

Fix patch:
diff --git a/docs/content/migration/v2-to-v3-details.md b/docs/content/migration/v2-to-v3-details.md
index fcbe58eea7..e40ab39f32 100644
--- a/docs/content/migration/v2-to-v3-details.md
+++ b/docs/content/migration/v2-to-v3-details.md
@@ -555,6 +555,16 @@ One should use the `ContentType` middleware to enable the `Content-Type` header
 
 ### Observability
 
+#### Open Connections Metric
+
+In v3, the open connections metric has been replaced with a global one because it was erroneously at the HTTP level, and providing misleading information.
+While previously produced at the entryPoint, router, and service levels, it is now replaced with a global metric.
+The equivalent to `traefik_entrypoint_open_connections`, `traefik_router_open_connections` and `traefik_service_open_connections` is now `traefik_open_connections`.
+
+#### Configuration Reload Failures Metrics
+
+In v3, the `traefik_config_reloads_failure_total` and `traefik_config_last_reload_failure` metrics have been suppressed since they could not be implemented.
+
 #### gRPC Metrics
 
 In v3, the reported status code for gRPC requests is now the value of the `Grpc-Status` header.


Test patch: