Move settings metrics to instrumentation class

What does this MR do and why?

Related to #424980 (closed).

This migrates a bunch of Service Ping metrics that read application settings in Gitlab::UsageData to use the metric instrumentation framework.

In order to reuse the instrumentation class, a method name can be specified in metric definitions that will be called on Gitlab::CurrentSettings.

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Before	After

How to set up and validate locally

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