all: Become a Go module (fixes #5148) (#5384)

* go mod init; rm -rf vendor

* tweak proto files and generation

* go mod vendor

* clean up build.go

* protobuf literals in tests

* downgrade gogo/protobuf

vendor/github.com/prometheus/common/NOTICE

@@ -0,0 +1,5 @@
+Common libraries shared by Prometheus Go components.
+Copyright 2015 The Prometheus Authors
+
+This product includes software developed at
+SoundCloud Ltd. (http://soundcloud.com/).