[#451] metrics: Move to internal

`metrics` don't look like something others want to import.

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

internal/metrics/types.go

@@ -0,0 +1,17 @@
+package metrics
+
+import (
+	"strconv"
+)
+
+type NullBool struct {
+	Bool  bool
+	Valid bool // Valid is true if Bool is not NULL
+}
+
+func (v NullBool) String() string {
+	if !v.Valid {
+		return ""
+	}
+	return strconv.FormatBool(v.Bool)
+}