[PROPOSAL] Test performance of basic Apache Beam operations

[Łukasz Gajowy]

Hi all,

since Synthetic Sources API has been introduced in Java and Python SDK, it
can be used to test some basic Apache Beam operations (i.e. GroupByKey,
CoGroupByKey Combine, ParDo and ParDo with SideInput) in terms of
performance. This, in brief, is why we'd like to share the below proposal:

*https://docs.google.com/document/d/1PuIQv4v06eosKKwT76u7S6IP88AnXhTf870Rcj1AHt4/edit?usp=sharing
<https://docs.google.com/document/d/1PuIQv4v06eosKKwT76u7S6IP88AnXhTf870Rcj1AHt4/edit?usp=sharing>*

Let us know what you think in the document's comments. Thank you in advance
for all the feedback!

Łukasz