[BEAM-13001] collect DoFn metrics for Combine (#15911)
[BEAM-13001] collect DoFn metrics for Combine (#15911)


lukecwik/incubator-beam,robertwb/incubator-beam,robertwb/incubator-beam,chamikaramj/beam,lukecwik/incubator-beam,robertwb/incubator-beam,robertwb/incubator-beam,apache/beam,apache/beam,chamikaramj/beam,apache/beam,robertwb/incubator-beam,lukecwik/incubator-beam,apache/beam,robertwb/incubator-beam,apache/beam,apache/beam,apache/beam,lukecwik/incubator-beam,robertwb/incubator-beam,chamikaramj/beam,apache/beam,chamikaramj/beam,lukecwik/incubator-beam,chamikaramj/beam,robertwb/incubator-beam,apache/beam,lukecwik/incubator-beam,chamikaramj/beam,apache/beam,chamikaramj/beam,robertwb/incubator-beam,lukecwik/incubator-beam,chamikaramj/beam,apache/beam,lukecwik/incubator-beam,chamikaramj/beam,robertwb/incubator-beam,lukecwik/incubator-beam,lukecwik/incubator-beam,chamikaramj/beam