Replace multiListerWatcher with independent listWatchers per namespace

[fpetkovski]

The multiListerWatcher is a composite object encapsulating multiple
ListerWatchers and implements the ListerWatcher interface.
With the current implementation, when an individual lister fails, the
entire List operation fails. This causes no metrics to be shown when KSM
has no permissions to a single namespace.

In addition to this, the multiListerWatcher takes advantage of internal
implementation details if the client-go library by modifiying and
relying on the ResourceVersion metadata field. This introduces a bug
where reconnecting to the API server will break the multiListerWatcher
completely.

This commit replaces the multiListerWatcher with individual
ListerWatchers per each configured namespace, resolving both issues.
It introduces a MetricsWriter interface to writes out metrics from stores,
and implements it in the MetricsStore struct. It also introduces a MultiNamespaceMetricsWriter
struct implementing MetricsWriter to allow writing out metrics from multiple
related stores (stores holding data for objects of the same kind but different namespace).

What this PR does / why we need it:
When KSM has no permissions for resources in at least one namespace, it will not export any metrics on the /metrics endpoint.

Which issue(s) this PR fixes:
Fixes #1413

Fixes a bug where KSM did not export any metrics when it had no permissions for resources in at least one namespace

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[mrueg]

/assign @tariq1890

we might want to backport this to release-2.1 as well.

[brancz]

Nice work!

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: brancz, dgrisonnet, fpetkovski, mrueg

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[brancz]

I have a suspicion that this might also help with #694