hostinfo: detect Heroku Dyno.

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

control/controlclient/hostinfo_linux.go

@@ -65,6 +65,9 @@ func osVersionLinux() string {
 	if inAwsLambda() {
 		attrBuf.WriteString("; env=lm")
 	}
+	if inHerokuDyno() {
+		attrBuf.WriteString("; env=hr")
+	}
 	attr := attrBuf.String()
 
 	id := m["ID"]
@@ -135,3 +138,10 @@ func inAwsLambda() bool {
 	}
 	return false
 }
+func inHerokuDyno() bool {
+	// https://devcenter.heroku.com/articles/dynos#local-environment-variables
+	if os.Getenv("PORT") != "" && os.Getenv("DYNO") != "" {
+		return true
+	}
+	return false
+}