Count should return a more specific type than PTransform

As `Count.globally()` and other static factory methods return objects of type `PTransform`, there is no way to communicate to the underlying `Combine` that we wish to have no default values, or to read the count as a `SingletonView` via the combiner.

Imported from Jira [BEAM-2501](https://issues.apache.org/jira/browse/BEAM-2501). Original Jira may contain additional context.
Reported by: tgroh.

Start by examining Count.globally(), the other static factory methods, PTransform, and the underlying Combine behavior described in the issue. Determine the intended more-specific return type and how it should expose no-default-value and SingletonView behavior; done requires a decided API design and corresponding coverage for those use cases.

Written by the indexing model from the issue text.