Serializing/Deserializing Logging and Metrics configs

[nachocano]

• Moving to pkg a way to serialize/deserialize logging and metric configMaps. This is a modified version of the code written by @n3wscott here, but as will be used in many places, we are moving it down to pkg.

[nachocano]

/assign @n3wscott

[knative-metrics-robot]

The following is the coverage report on pkg/.
Say /test pull-knative-pkg-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
logging/config.go	88.9%	84.8%	-4.1
metrics/config.go	82.5%	82.6%	0.1

[n3wscott]

/lgtm

[nachocano]

@yanweiguo this one is ready for approval. I think @n3wscott doesn't have approval in these folder

[yanweiguo]

/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nachocano, yanweiguo

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• logging/OWNERS [yanweiguo]
• metrics/OWNERS [yanweiguo]

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