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/rcrowley/go-metrics/.gitignore

@@ -0,0 +1,9 @@
+*.[68]
+*.a
+*.out
+*.swp
+_obj
+_testmain.go
+cmd/metrics-bench/metrics-bench
+cmd/metrics-example/metrics-example
+cmd/never-read/never-read