Add MetricRegistry factory

[derjust]

This patchset allows to configure a different way on how
MetricRegistry objects are created:

Up to now these were created within money itself with no exposition to
the rest of the system. That makes it impossible to register other
Dropwizzard reporters or exporters on it.

With this patchset it is now possible to provide a custom MetricRegistry
factory and do the required initialization of/with the MetricRegistry
object.

It is compatible to Java so even Java clients (using the Scala money)
can implement and configure a custom factory.

[codecov-io]

Current coverage is 95.33%

Merging #35 into master will increase coverage by +0.05% as of 1af58d3

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  Files           35      36     +1
  Stmts          891     901    +10
  Branches        91      91       
  Methods          0       0       
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+ Hit            849     859    +10
  Partial          0       0       
  Missed          42      42       

Review entire Coverage Diff as of 1af58d3

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[kristomasette]

I am thinking since we defined the Default in reference.conf and ensure the DefaultMetricRegistryFactory is in the classpath by including it in the library we should probably log loudly and move on if there is an error.

There should be no way we hit that Fauilure case unless something is really wrong right?

[derjust]

Got your point.
Followed the pattern from the overall Money initialization: https://github.com/Comcast/money/pull/19/files#r44557079

[kristomasette]

ok, I guess it makes sense to stop and yell loudly

[kristomasette]

actually can this lazy val realFactory be at the MetricRegistryFactory level?

That way it gets built once the first time its accessed.

[kristomasette]

👍