Added FailedQueryCounter inc()

Hello @dewey, Thanks for your review comments. 

As per your previous comments, I have added Inc() and yes this gives more info on number of Query failures. This info really helps in debugging  and understanding the failures.

Please review and let me know if any other suggestion.

Thanks,
Santoshkumar Patil

Author: santubvb

query.go

@@ -67,6 +67,7 @@ func (q *Query) Run(conn *connection) error {
 			failedScrapes.WithLabelValues(conn.driver, conn.host, conn.database, conn.user, q.jobName, q.Name).Set(0.0)
 		} else {
 			failedScrapes.WithLabelValues(conn.driver, conn.host, conn.database, conn.user, q.jobName, q.Name).Set(1.0)
+			failedQueryCounter.WithLabelValues(q.jobName, q.Name).Inc()
 			return fmt.Errorf("zero rows returned")
 		}
 	}