prometheus: Add UpdateFromSet to MetricSet

Allows a set to updated by another.

prometheus/metricset.go

@@ -49,6 +49,13 @@ func (set *MetricSet) Update(m *Metric) {
 	set.metrics[metricKey(m)] = m
 }
 
+// UpdateFromSet updates the values in the set from another MetricSet.
+func (set *MetricSet) UpdateFromSet(other *MetricSet) {
+	for _, m := range other.All() {
+		set.Update(m)
+	}
+}
+
 // ToBytes serialise the metrics contained in the MetricSet to the
 // Prometheus exposition format.
 func (set *MetricSet) ToBytes() []byte {

prometheus/metricset_small_test.go

@@ -155,6 +155,55 @@ func TestUpdateLabelOrdering(t *testing.T) {
 	assert.Equal(t, set.All(), []*prometheus.Metric{m1})
 }
 
+func TestUpdateFromSet(t *testing.T) {
+	// set0 has 2 metrics
+	set0 := prometheus.NewMetricSet()
+	m0 := &prometheus.Metric{
+		Name: []byte("uptime"),
+		Labels: prometheus.LabelPairs{
+			{Name: []byte("host"), Value: []byte("nyc01")},
+		},
+		Value:        222,
+		Milliseconds: 99,
+	}
+	set0.Update(m0)
+	set0.Update(&prometheus.Metric{
+		Name: []byte("temp"),
+		Labels: prometheus.LabelPairs{
+			{Name: []byte("host"), Value: []byte("nyc02")},
+			{Name: []byte("core"), Value: []byte("0")},
+		},
+		Value:        55,
+		Milliseconds: 100,
+	})
+
+	// set1 overwrites one item in set0 and introduces a new one.
+	set1 := prometheus.NewMetricSet()
+	m1 := &prometheus.Metric{
+		Name: []byte("temp"),
+		Labels: prometheus.LabelPairs{
+			{Name: []byte("host"), Value: []byte("nyc02")},
+			{Name: []byte("core"), Value: []byte("0")},
+		},
+		Value:        66,
+		Milliseconds: 111,
+	}
+	set1.Update(m1)
+	m2 := &prometheus.Metric{
+		Name: []byte("uptime"),
+		Labels: prometheus.LabelPairs{
+			{Name: []byte("host"), Value: []byte("nyc02")},
+		},
+		Value:        1234,
+		Milliseconds: 222,
+	}
+	set1.Update(m2)
+
+	set0.UpdateFromSet(set1)
+
+	assert.ElementsMatch(t, set0.All(), []*prometheus.Metric{m0, m1, m2})
+}
+
 func TestToBytes(t *testing.T) {
 	set := prometheus.NewMetricSet()