Add tagging script

Add a script that will create and push tags
to the upstream repo, for each module.

CONTRIBUTING.md

@@ -82,4 +82,9 @@ adequately cover the code using `go test -cover`.
 2. Update CHANGELOG.md, adding a new version heading and changing the base tag of the Unreleased comparison URL
 3. Merge changes into github.com/elastic/apm-agent-go@master
 4. Create tags: vN.N.N, and module/$MODULE/vN.N.N for each instrumentation module
+
+	scripts/tagversion.sh
+
 5. Create release on GitHub
+
+	hub release -d vN.N.N

scripts/tagversion.sh

@@ -0,0 +1,23 @@
+#!/bin/sh
+set -e
+
+export GO111MODULE=on
+
+prefix=go.elastic.co/apm
+version=$(sed 's@^\s*AgentVersion = "\(.*\)"$@\1@;t;d' version.go)
+modules=$(for dir in $(./scripts/moduledirs.sh); do (cd $dir && go list -m); done | grep $prefix/)
+
+echo "# Create tags"
+for m in "" $modules; do
+	p=$(echo $m | sed "s@^$prefix/\(.\{0,\}\)@\1/@")
+	echo git tag -s ${p}v$version -m v$version
+done
+
+echo
+echo "# Push tags"
+echo -n git push upstream
+for m in "" $modules; do
+	p=$(echo $m | sed "s@^$prefix/\(.\{0,\}\)@\1/@")
+	echo -n " ${p}v$version"
+done
+echo