feat: Ignore ready and stats listener metrics in shutdown manager calculation

[zirain]

fixes: #7947

I'm not sure if this worked on every cases, so users're able to fallback to use server.total_connections with following:

apiVersion: gateway.networking.k8s.io/v1
kind: GatewayClass
metadata:
  name: eg-with-ep
spec:
  controllerName: gateway.envoyproxy.io/gatewayclass-controller
  parametersRef:
    group: gateway.envoyproxy.io
    kind: EnvoyProxy
    name: proxy-config
    namespace: envoy-gateway-system
---
apiVersion: gateway.networking.k8s.io/v1
kind: Gateway
metadata:
  name: eg-with-ep
spec:
  gatewayClassName: eg-with-ep
  listeners:
    - name: http
      protocol: HTTP
      port: 80
---
apiVersion: gateway.envoyproxy.io/v1alpha1
kind: EnvoyProxy
metadata:
  name: proxy-config
  namespace: envoy-gateway-system
spec:
  provider:
    type: Kubernetes
    kubernetes:
      envoyDeployment:
        patch:
          type: StrategicMerge
          value:
            spec:
              template:
                spec:
                  containers:
                    - name: shutdown-manager
                      env:
                        - name: "USE_SERVER_CONNECTIONS"
                          value: "true"

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[codecov]

Codecov Report

❌ Patch coverage is 69.23077% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.87%. Comparing base (b28b77c) to head (2e002c6).

Files with missing lines	Patch %	Lines
internal/cmd/envoy/shutdown_manager.go	69.23%	13 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7985   +/-   ##
=======================================
  Coverage   72.87%   72.87%           
=======================================
  Files         237      237           
  Lines       35509    35534   +25     
=======================================
+ Hits        25877    25896   +19     
- Misses       7792     7793    +1     
- Partials     1840     1845    +5     

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