Add context to "duplicate label names" to enable debugging (#1177)

* Add context to "duplicate label names" to enable debugging

Signed-off-by: SpencerMalone <malone.spencer@gmail.com>

* Remove unused errors package import.

Signed-off-by: SpencerMalone <malone.spencer@gmail.com>

Signed-off-by: SpencerMalone <malone.spencer@gmail.com>

prometheus/desc.go

@@ -14,7 +14,6 @@
 package prometheus
 
 import (
-	"errors"
 	"fmt"
 	"sort"
 	"strings"
@@ -127,7 +126,7 @@ func NewDesc(fqName, help string, variableLabels []string, constLabels Labels) *
 		labelNameSet[labelName] = struct{}{}
 	}
 	if len(labelNames) != len(labelNameSet) {
-		d.err = errors.New("duplicate label names")
+		d.err = fmt.Errorf("duplicate label names in constant and variable labels for metric %q", fqName)
 		return d
 	}