Improve MetricRegistryFactory configuration

[derjust]

The current appraoch requires two properties to be set to enable a
custom MetricRegistryFactory:

  metrics-registry {
    class-name = "com.acme.CustomMetricRegistryFactory"
  }

  emitter {
    emitters = [
      {
        name = "span-metrics-emitter"
        class-name = "com.comcast.money.metrics.SpanMetricsCollector"
        subscriptions = [Trace]
        configuration = {
          metrics-registry {
            class-name = "com.acme.CustomMetricRegistryFactory"
          }
        }
      }
    ]
  }

With this change, the configuration simplifies down to

  metrics-registry {
     class-name = "com.acme.CustomMetricRegistryFactory"
     configuration = {
     }
  }

for the money system and providing a custom 'configuration' section for the factory.

Also, as it is now an Akka extension, the factory is created only once for the whole system.

[codecov-io]

Current coverage is 95.23%

Merging #39 into master will decrease coverage by -0.10% as of 6d57443

@@            master     #39   diff @@
======================================
  Files           36      36       
  Stmts          901     902     +1
  Branches        91      91       
  Methods          0       0       
======================================
  Hit            859     859       
  Partial          0       0       
- Missed          42      43     +1

Review entire Coverage Diff as of 6d57443

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[kristomasette]

should this be: getLogger("com.comcast.money.metrics.MetricRegistryExtensionImpl")

[kristomasette]

one minor nit to pick, but otherwise, looks good

[paulcleary]

can you use /* instead of multiple lines of //

[derjust]

Addressed @kristomasette and @paulcleary comments

[kristomasette]

👍