operator/eni: fix panic if metrics are not enabled
operator/eni: fix panic if metrics are not enabled

Fix the following panic in the operator if metrics are not enabled:

```
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x8d9eac]

goroutine 1 [running]:
github.com/cilium/cilium/vendor/github.com/prometheus/client_golang/prometheus.(*Registry).Register(0x0, 0x2288ba0, 0xc00000e880, 0x0, 0x0)
        /go/src/github.com/cilium/cilium/vendor/github.com/prometheus/client_golang/prometheus/registry.go:276 +0x15c
github.com/cilium/cilium/vendor/github.com/prometheus/client_golang/prometheus.(*Registry).MustRegister(0x0, 0xc00055c648, 0x1, 0x1)
        /go/src/github.com/cilium/cilium/vendor/github.com/prometheus/client_golang/prometheus/registry.go:390 +0x76
github.com/cilium/cilium/pkg/aws/eni/metrics.NewPrometheusMetrics(0x1f1353b, 0xf, 0x0, 0x1)
        /go/src/github.com/cilium/cilium/pkg/aws/eni/metrics/metrics.go:109 +0xb39
main.startENIAllocator(0x4034000000000000, 0x4, 0x10, 0x4)
        /go/src/github.com/cilium/cilium/operator/eni.go:116 +0x5bb
main.runOperator(0x3946ee0)
        /go/src/github.com/cilium/cilium/operator/main.go:221 +0x154e
main.glob..func1(0x3946ee0, 0xc0004342d0, 0x0, 0x3)
        /go/src/github.com/cilium/cilium/operator/main.go:51 +0x2b
github.com/cilium/cilium/vendor/github.com/spf13/cobra.(*Command).execute(0x3946ee0, 0xc00003a090, 0x3, 0x3, 0x3946ee0, 0xc00003a090)
        /go/src/github.com/cilium/cilium/vendor/github.com/spf13/cobra/command.go:766 +0x2ae
github.com/cilium/cilium/vendor/github.com/spf13/cobra.(*Command).ExecuteC(0x3946ee0, 0x0, 0x0, 0x0)
        /go/src/github.com/cilium/cilium/vendor/github.com/spf13/cobra/command.go:850 +0x2fc
github.com/cilium/cilium/vendor/github.com/spf13/cobra.(*Command).Execute(...)
        /go/src/github.com/cilium/cilium/vendor/github.com/spf13/cobra/command.go:800
main.main()
        /go/src/github.com/cilium/cilium/operator/main.go:90 +0x1f0
```

If metrics are not enabled, provide an empty type which implements interfaces
needed by the ENI and EC2 subsystems within Cilium.

Signed-off by: Ian Vernon <ian@cilium.io>

cilium/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium-team/cilium,tgraf/cilium,tklauser/cilium,tgraf/cilium,tgraf/cilium,michi-covalent/cilium,cilium-team/cilium,cilium/cilium,tgraf/cilium,tklauser/cilium,tklauser/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium,tgraf/cilium,tklauser/cilium,tklauser/cilium,michi-covalent/cilium,cilium/cilium,tgraf/cilium