Rename Go version tag from version to go_version (#122)

Most of our services will set a version tag that represents the version of their service. However, the Go stats reported by the `procstats` package will overwrite this tag with the current Go runtime version.

This becomes a bigger issue when scraping these stats via Prometheus, since it will consider a sample as invalid if labels are not unique:

![image](https://user-images.githubusercontent.com/2907397/89065716-3345d580-d33a-11ea-93e7-ab4833388980.png)

This PR makes a breaking change to resolve this issue, by renaming the tag that specifics the Go runtime version from `version` to `go_version`.

README.md

@@ -181,6 +181,10 @@ Monitoring
 
 ### Processes
 
+> 🚧 Go metrics reported with the `procstats` package were previously tagged with a
+> `version` label that reported the Go runtime version. This label was renamed to
+> `go_version` in v4.6.0.
+
 The
 [github.com/segmentio/stats/procstats](https://godoc.org/github.com/segmentio/stats/procstats)
 package exposes an API for creating a statistics collector on local processes.

procstats/go.go

@@ -23,8 +23,8 @@ func init() {
 
 // GoMetrics is a metric collector that reports metrics from the Go runtime.
 type GoMetrics struct {
-	engine  *stats.Engine
-	version string `tag:"version"`
+	engine    *stats.Engine
+	goVersion string `tag:"go_version"`
 
 	runtime struct {
 		// Runtime info.
@@ -113,8 +113,8 @@ func NewGoMetrics() *GoMetrics {
 // metrics on eng.
 func NewGoMetricsWith(eng *stats.Engine) *GoMetrics {
 	g := &GoMetrics{
-		engine:  eng,
-		version: runtime.Version(),
+		engine:    eng,
+		goVersion: runtime.Version(),
 	}
 
 	g.memstats.total.memtype = "total"