Adjust collector to use separate connection per scrape (#931)

Fixes #921

Signed-off-by: Joe Adams <github@joeadams.io>

collector/instance.go

@@ -43,6 +43,13 @@ func newInstance(dsn string) (*instance, error) {
 	return i, nil
 }
 
+// copy returns a copy of the instance.
+func (i *instance) copy() *instance {
+	return &instance{
+		dsn: i.dsn,
+	}
+}
+
 func (i *instance) setup() error {
 	db, err := sql.Open("postgres", i.dsn)
 	if err != nil {