Add missing return values in MetricMaker
Add missing return values in MetricMaker

When new[Callback|Constant]Metric methods are called as an opposite to
the remaining methods they don't return the RegistrationHandle. As a
result it is no longer possible to de-register metrics created with that
methods.

Release-Notes: skip
Change-Id: I06d48535702a8acd0bcb4e1315a35b3c1d1768cb

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