remove GetExemplars function

Signed-off-by: William Perron <william.perron@shopify.com>

Author: wperron

prometheus/histogram.go

@@ -611,13 +611,6 @@ func (h *constHistogram) Write(out *dto.Metric) error {
 return nil
 }
 
-func (h *constHistogram) GetExemplars() []*dto.Exemplar {
-if h != nil {
-return h.exemplars
-}
-return nil
-}
-
 // NewConstHistogram returns a metric representing a Prometheus histogram with
 // fixed values for the count, sum, and bucket counts. As those parameters
 // cannot be changed, the returned value does not implement the Histogram