sweet: make etcd a default benchmark

Change-Id: I91204b6500bac3d45e71f44eca553c5798289bb9
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/493919
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>