Modernize invertergui: MQTT write support, HA integration, UI updates

vendor/github.com/prometheus/client_golang/prometheus/metric.go

@@ -108,15 +108,23 @@ func BuildFQName(namespace, subsystem, name string) string {
 	if name == "" {
 		return ""
 	}
-	switch {
-	case namespace != "" && subsystem != "":
-		return strings.Join([]string{namespace, subsystem, name}, "_")
-	case namespace != "":
-		return strings.Join([]string{namespace, name}, "_")
-	case subsystem != "":
-		return strings.Join([]string{subsystem, name}, "_")
+
+	sb := strings.Builder{}
+	sb.Grow(len(namespace) + len(subsystem) + len(name) + 2)
+
+	if namespace != "" {
+		sb.WriteString(namespace)
+		sb.WriteString("_")
 	}
-	return name
+
+	if subsystem != "" {
+		sb.WriteString(subsystem)
+		sb.WriteString("_")
+	}
+
+	sb.WriteString(name)
+
+	return sb.String()
 }
 
 type invalidMetric struct {
@@ -178,21 +186,31 @@ func (m *withExemplarsMetric) Write(pb *dto.Metric) error {
 	case pb.Counter != nil:
 		pb.Counter.Exemplar = m.exemplars[len(m.exemplars)-1]
 	case pb.Histogram != nil:
+		h := pb.Histogram
 		for _, e := range m.exemplars {
-			// pb.Histogram.Bucket are sorted by UpperBound.
-			i := sort.Search(len(pb.Histogram.Bucket), func(i int) bool {
-				return pb.Histogram.Bucket[i].GetUpperBound() >= e.GetValue()
+			if (h.GetZeroThreshold() != 0 || h.GetZeroCount() != 0 ||
+				len(h.PositiveSpan) != 0 || len(h.NegativeSpan) != 0) &&
+				e.GetTimestamp() != nil {
+				h.Exemplars = append(h.Exemplars, e)
+				if len(h.Bucket) == 0 {
+					// Don't proceed to classic buckets if there are none.
+					continue
+				}
+			}
+			// h.Bucket are sorted by UpperBound.
+			i := sort.Search(len(h.Bucket), func(i int) bool {
+				return h.Bucket[i].GetUpperBound() >= e.GetValue()
 			})
-			if i < len(pb.Histogram.Bucket) {
-				pb.Histogram.Bucket[i].Exemplar = e
+			if i < len(h.Bucket) {
+				h.Bucket[i].Exemplar = e
 			} else {
 				// The +Inf bucket should be explicitly added if there is an exemplar for it, similar to non-const histogram logic in https://github.com/prometheus/client_golang/blob/main/prometheus/histogram.go#L357-L365.
 				b := &dto.Bucket{
-					CumulativeCount: proto.Uint64(pb.Histogram.GetSampleCount()),
+					CumulativeCount: proto.Uint64(h.GetSampleCount()),
 					UpperBound:      proto.Float64(math.Inf(1)),
 					Exemplar:        e,
 				}
-				pb.Histogram.Bucket = append(pb.Histogram.Bucket, b)
+				h.Bucket = append(h.Bucket, b)
 			}
 		}
 	default:
@@ -219,6 +237,7 @@ type Exemplar struct {
 // Only last applicable exemplar is injected from the list.
 // For example for Counter it means last exemplar is injected.
 // For Histogram, it means last applicable exemplar for each bucket is injected.
+// For a Native Histogram, all valid exemplars are injected.
 //
 // NewMetricWithExemplars works best with MustNewConstMetric and
 // MustNewConstHistogram, see example.
@@ -234,7 +253,7 @@ func NewMetricWithExemplars(m Metric, exemplars ...Exemplar) (Metric, error) {
 	)
 	for i, e := range exemplars {
 		ts := e.Timestamp
-		if ts == (time.Time{}) {
+		if ts.IsZero() {
 			ts = now
 		}
 		exs[i], err = newExemplar(e.Value, ts, e.Labels)