Allow mechanism for only pulling resources in the check's namespace

[metrictwo]

Currently, plugins will pull pods and services at the cluster scope and then prune them against namespace-limiting options (-i|--include-namespace and -n|--exclude-namespace). This assumes that the pod running the check can read resources at the cluster scope, preventing multi-tenant cluster users from running their own per-namespace clients (well, without doling out overly-generous RBAC perms, that is). A few possible options:

• Modify --include-namespace to explicitly pull resources from the provided namespace(s) individually, rather than pulling cluster-wide and then pruning.
• Add a flag specifying cluster-wide vs namespaced usage. Examples:
  • --cluster-wide=<bool> (true by default)
  • --namespaced=<bool> (false by default)
• Add a --namespace [NS] option to specify the namespace in which to operate. Perhaps the argument could be optional, with the current namespace used when not specified.

I am happy to perform this work, although input on a preferred route would be appreciated.