Add the instance struct to handle connections (#859)

The intent is to use the instance struct to hold the connection
to the database as well as metadata about the instance:
- version
- flavor (mariadb or mysql)

Change is similar to prometheus-community/postgres_exporter#785

Signed-off-by: Vlad Gusev <vlad.esten@gmail.com>

collector/engine_innodb_test.go

@@ -152,10 +152,10 @@ END OF INNODB MONITOR OUTPUT
 	rows := sqlmock.NewRows(columns).AddRow("InnoDB", "", sample)
 
 	mock.ExpectQuery(sanitizeQuery(engineInnodbStatusQuery)).WillReturnRows(rows)
-
+	inst := &instance{db: db}
 	ch := make(chan prometheus.Metric)
 	go func() {
-		if err = (ScrapeEngineInnodbStatus{}).Scrape(context.Background(), db, ch, log.NewNopLogger()); err != nil {
+		if err = (ScrapeEngineInnodbStatus{}).Scrape(context.Background(), inst, ch, log.NewNopLogger()); err != nil {
 			t.Errorf("error calling function on test: %s", err)
 		}
 		close(ch)