metrics: use buildinfo collector from new collectors pkg (#4187)

Signed-off-by: Dave Henderson <dhenderson@gmail.com>

metrics.go

@@ -6,12 +6,13 @@ import (
 	"strings"
 
 	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/collectors"
 	"github.com/prometheus/client_golang/prometheus/promauto"
 )
 
 // define and register the metrics used in this package.
 func init() {
-	prometheus.MustRegister(prometheus.NewBuildInfoCollector())
+	prometheus.MustRegister(collectors.NewBuildInfoCollector())
 
 	const ns, sub = "caddy", "admin"