Remove unnecessary check if label is nil in observeWithExemplar instrumentation

Author: dlituyev

prometheus/promhttp/instrument_server.go

@@ -37,10 +37,6 @@ func observeWithExemplar(obs prometheus.Observer, val float64, labels map[string
 // addWithExemplar is a wrapper for [prometheus.ExemplarAdder.AddWithExemplar],
 // which falls back to [prometheus.Counter.Add] if no labels are provided.
 func addWithExemplar(obs prometheus.Counter, val float64, labels map[string]string) {
-	if labels == nil {
-		obs.Add(val)
-		return
-	}
 	obs.(prometheus.ExemplarAdder).AddWithExemplar(val, labels)
 }