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/validate.sh

@@ -0,0 +1,10 @@
+#!/bin/bash
+
+set -e
+
+# check there are no formatting issues
+GOFMT_LINES=`gofmt -l . | wc -l | xargs`
+test $GOFMT_LINES -eq 0 || echo "gofmt needs to be run, ${GOFMT_LINES} files have issues"
+
+# run the tests for the root package
+go test -race .