Add a new metrics to expose local cached items size

[halfcrazy]

Backgroud
In controller-runtime, the default behaviour is to cache all data using client.Get unless using a read-only client or custom cache policies, e.g., ClientDisableCacheFor, NewCache. Therefore, using client.Get on a specific pod in a large cluster with many pods may lead to high memory usage.

Proposal
Add a new metric to expose the local cached items' size. Then we can analyze the high memory usage caused by the misuse cache policy.

[halfcrazy]

#3054