[notify] Add PubSub message metric
[notify] Add PubSub message metric

Add a counter of buildbucket pubsub messages with a string "status" field
which can take values "success", "permanent-failure" and "transient-failure".
An alert for "permanent-failure" will be added.

Bug: 931518
Change-Id: I88e3fcd0092dd8266f24a911435e35c3fac9311b
Reviewed-on: https://chromium-review.googlesource.com/c/1470612
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
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