[bug] We cannot see any metrics about gRPC which used in cri package

[fengzixu]

Ⅰ. Issue Description

When I resolved this issue #2395 , I revised the logic about metrics in cri package. I found that GRPCMetrics(https://github.com/alibaba/pouch/blob/master/cri/metrics/metrics.go#L23) was used in https://github.com/alibaba/pouch/blob/1f7dbf60d406210c65fb96ea4682f6d04f97773e/cri/v1alpha2/service/cri.go#L21 . But, the GRPCMetrics wasn't registered to defaultPrometheusRegistry or customPrometheusRegistry.

Ⅱ. Describe what happened

1. GRPCMetrics wasn't registered
2. Not found any metric about gRPC

Ⅲ. Describe what you expected to happen

We should register GRPCMetrics to registry about Prometheus. After that, we can see the metrics through curl http://xxx/metrics

Ⅳ. How to reproduce it (as minimally and precisely as possible)

If someone can help me to identify this bug, I will revise it together with #2395.

Ⅴ. Anything else we need to know?

Ⅵ. Environment:

• pouch version (use pouch version):
• OS (e.g. from /etc/os-release):
• Kernel (e.g. uname -a):
• Install tools:
• Others:

[starnop]

At present, GRPCMetrics is not necessary actually, so delete it firstly. Sorry for forgot to add a TODO tag. Maybe you can help add it or make it registered. THX.

[fengzixu]

@starnop OK. I will delete it.