[buildbucket] Update builder stats when scheduling builds
[buildbucket] Update builder stats when scheduling builds

Bug: 1042991
Change-Id: Ibc72a28c2f20117e1c500423fc28d05b0a6e019e
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2808274
Reviewed-by: Yuanjun Huang <a5f4b8445ce5e958d19c0a7caeadcd07c9d18770@google.com>
Commit-Queue: smut <95deb7aa15d4c615299e47e7688551456568b1fb@google.com>

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