Remove unnecessary check if label is nil in observeWithExemplar instrumentation

[bwplotka]

Tiny task to do: I noticed that

client_golang/prometheus/promhttp/instrument_server.go

Line 33 in 3d2cf0b

func observeWithExemplar(obs prometheus.Observer, val float64, labels map[string]string) {

is kind of noop as underlying ObserveWithExemplar and AddWithExemplar has already such characteristics. Let's remove unnecessary middle functions.

[bwplotka]

Fixed by #1235