[filegraph] Print graph size
[filegraph] Print graph size

Print the graph size in the E2E benchmark.

Bug: 1136280
Change-Id: I99b8f053462782b942b2b771b0999c1d11db44a6
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2539886
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Commit-Queue: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go