Dropped windows counters / log prints no longer working

In https://github.com/apache/beam/pull/2838 aggregators were removed from Spark runner, this caused regression around dropped windows counters and logs.

`CounterCell` instances are created ad hoc instead of using the `Metrics` class static factory methods: [SparkGroupAlsoByWindowViaWindowSet.java#L213-L219](https://github.com/apache/beam/blob/v2.1.0/runners/spark/src/main/java/org/apache/beam/runners/spark/stateful/SparkGroupAlsoByWindowViaWindowSet.java#L213-L219)
Context of where the metrics are reported isn't taken into account, and since these counters are being passed to a lazily evaluated iterator [SparkGroupAlsoByWindowViaWindowSet.java#L221-L223](https://github.com/apache/beam/blob/v2.1.0/runners/spark/src/main/java/org/apache/beam/runners/spark/stateful/SparkGroupAlsoByWindowViaWindowSet.java#L221-L223) the subsequent code which looks at the counters is always looking at these counters immediately after initialization, before they are populated, so these prints will never happen since the conditional statements do not check on the right counters [SparkGroupAlsoByWindowViaWindowSet.java#L323-L333](https://github.com/apache/beam/blob/v2.1.0/runners/spark/src/main/java/org/apache/beam/runners/spark/stateful/SparkGroupAlsoByWindowViaWindowSet.java#L323-L333).
What we want is these counts exposed as metrics as well as logs.

Additionally, `org.apache.beam.runners.core.LateDataUtils#dropExpiredWindows` now takes a `CounterCell` as a parameter, which is a class for metrics implementation and should generally not be used elsewhere (this is also mentioned in its Javadoc), we should look into changing this method to use something else and perhaps make `CounterCell` and similar classes package private (And change runner code which uses these to be in the same package).

Imported from Jira [BEAM-2812](https://issues.apache.org/jira/browse/BEAM-2812). Original Jira may contain additional context.
Reported by: aviemzur.

Start with runners/spark/src/main/java/org/apache/beam/runners/spark/stateful/SparkGroupAlsoByWindowViaWindowSet.java, examining the CounterCell creation, lazy iterator, and conditional logging around the referenced lines. Then read org.apache.beam.runners.core.LateDataUtils#dropExpiredWindows and its Javadoc. Done means dropped-window counts are exposed as metrics and logs, while the utility no longer depends directly on CounterCell.

Written by the indexing model from the issue text.