translations.pack_combiners optimizer causes breaking change to metrics API

The translations.pack_combiners optimizer causes a breaking change in the public metrics API. The issue arises because metrics are keyed and queryable by step name, and the step name can change after combiner packing. Suppose we have a pipeline that looks like `pipeline | CombinePerKey(combinefn_1); pipeline | CombinePerKey(combinefn_2)` and both combinefn_1 and combinefn_2 increment the same counter per input element. Previously, the result would have two counters, one each for step combinefn_1 and combinefn_2; both will have value num_input_elements. After combiner packing, the result will have one counter for Packed[combinefn_1, combinefn] with value 2 * num_input_elements.

Unfortunately there is no easy fix for this because the runner has to somehow be aware that a step is a packed step and use the appropriate metrics container for the sub-step.

The short term workaround is to (1) add a note for 2.27 under known issues and (2) make this phase opt-in in 2.28.