Allow providing a custom MetricsFactory to Counter and friends (#172)

### Motivation:

To allow for things like parallel testing, it'd be useful if we can
explicitly provide a MetricsFactory to the existing Counter/... types.

### Modifications:

Added a `factory: MetricsFactory` parameter to all the initializers of
Counter/... types, and kept the existing methods that continue to
default to `MetricsSystem.factory`.

### Result:

Adopters can use a custom MetricsFactory explicitly passed in at metric
creation time. Existing adopters are not affected, unless you opt in,
you continue to use the global factory.

Author: czechboy0