perElement() can be used to count the number of occurrences of each
distinct element in the PCollection, perKey() can be used to count the
number of values per key, and globally() can be used to count the total
number of elements in a PCollection.
Count.PerElement<T> takes a PCollection<T> and returns a
PCollection<KV<T, Long>> representing a map from each distinct element of the input
PCollection to the number of times that element occurs in the input.