Buffer all the output in memory before writing it to the provided io.Writer in WritePrometheus

This should prevent from indefinite locking of internal structures in the case of laggy io.Writer

set.go

@@ -1,6 +1,7 @@
 package metrics
 
 import (
+	"bytes"
 	"fmt"
 	"io"
 	"sort"
@@ -29,6 +30,8 @@ func NewSet() *Set {
 
 // WritePrometheus writes all the metrics from s to w in Prometheus format.
 func (s *Set) WritePrometheus(w io.Writer) {
+	// Collect all the metrics in in-memory buffer in order to prevent from long locking due to slow w.
+	var bb bytes.Buffer
 	lessFunc := func(i, j int) bool {
 		return s.a[i].name < s.a[j].name
 	}
@@ -40,9 +43,10 @@ func (s *Set) WritePrometheus(w io.Writer) {
 		sort.Slice(s.a, lessFunc)
 	}
 	for _, nm := range s.a {
-		nm.metric.marshalTo(nm.name, w)
+		nm.metric.marshalTo(nm.name, &bb)
 	}
 	s.mu.Unlock()
+	w.Write(bb.Bytes())
 }
 
 // NewHistogram creates and returns new histogram in s with the given name.