Removing support for Go <=1.8, removing httprouter, adding new observe package

Extract GCP Monitoring (#192)

* Extract GCP Monitoring

* remove s

* add pkg doc

* return exporter

* return flush func

* try another approach

* bypass if check

* gcputil -> observe

* fix doc

* pr fixes

* fix gae metrics

* pr fixes

* pr fixes

* doc fixes

* better err msg

* log fix

* remove prom from observe

* sd brand

* fix build

* observe README