Map JSON properties as MQTT labels

.dockerignore

@@ -1,3 +1,4 @@
 bin/
 .git
-systemd/
+systemd/
+hack

Dockerfile

@@ -1,4 +1,10 @@
-FROM golang:1.23 as builder
+FROM golang:1.24 AS builder
+
+# enable cross-platform builds with CGO_ENABLED
+# I had to first compile without buildx for buildx to then work
+ENV CGO_ENABLED=1
+ENV GOOS=linux
+ENV GOARCH=amd64
 
 COPY . /build/mqtt2prometheus
 WORKDIR /build/mqtt2prometheus

config.yaml.dist

@@ -46,6 +46,8 @@ metrics:
     # A map of string to string for constant labels. This labels will be attached to every prometheus metric
     const_labels:
       sensor_type: dht22
+    inherit_labels:
+      - serialNumber
     # The name of the metric in prometheus
   - prom_name: humidity
     # The name of the metric in a MQTT JSON message

pkg/config/config.go

@@ -146,10 +146,10 @@ type MetricConfig struct {
 	ForceMonotonicy    bool                      `yaml:"force_monotonicy"`
 	ConstantLabels     map[string]string         `yaml:"const_labels"`
 	DynamicLabels      map[string]string         `yaml:"dynamic_labels"`
+	InheritLabels      []string                  `yaml:"inherit_labels"`
 	StringValueMapping *StringValueMappingConfig `yaml:"string_value_mapping"`
 	MQTTValueScale     float64                   `yaml:"mqtt_value_scale"`
-	// ErrorValue is used while error during value parsing
-	ErrorValue         *float64                  `yaml:"error_value"`
+	ErrorValue         *float64                  `yaml:"error_value"` // ErrorValue is used while error during value parsing
 }
 
 // StringValueMappingConfig defines the mapping from string to float
@@ -162,6 +162,7 @@ type StringValueMappingConfig struct {
 
 func (mc *MetricConfig) PrometheusDescription() *prometheus.Desc {
 	labels := append([]string{"sensor", "topic"}, mc.DynamicLabelsKeys()...)
+	labels = append(labels, mc.InheritLabels...)
 	return prometheus.NewDesc(
 		mc.PrometheusName, mc.Help, labels, mc.ConstantLabels,
 	)
@@ -257,7 +258,7 @@ func LoadConfig(configFile string, logger *zap.Logger) (Config, error) {
 			logger.Warn("string_value_mapping.error_value is deprecated: please use error_value at the metric level.", zap.String("prometheusName", m.PrometheusName), zap.String("MQTTName", m.MQTTName))
 		}
 
-		if m.Expression != "" && m.RawExpression != ""  {
+		if m.Expression != "" && m.RawExpression != "" {
 			return Config{}, fmt.Errorf("metric %s/%s: expression and raw_expression are mutually exclusive.", m.MQTTName, m.PrometheusName)
 		}
 	}

pkg/metrics/extractor.go

@@ -24,6 +24,7 @@ func NewJSONObjectExtractor(p Parser) Extractor {
 	return func(topic string, payload []byte, deviceID string) (MetricCollection, error) {
 		var mc MetricCollection
 		parsed := gojsonq.New(gojsonq.SetSeparator(p.separator)).FromString(string(payload))
+		rawPayload := gojsonq.New(gojsonq.SetSeparator(p.separator)).FromString(string(payload))
 
 		for path := range p.config() {
 			rawValue := parsed.Find(path)
@@ -33,12 +34,19 @@ func NewJSONObjectExtractor(p Parser) Extractor {
 			}
 
 			// Find all valid metric configs
+			// var labels map[string]string
 			for _, config := range p.findMetricConfigs(path, deviceID) {
 				id := metricID(topic, path, deviceID, config.PrometheusName)
 				m, err := p.parseMetric(config, id, rawValue)
 				if err != nil {
-					return nil, fmt.Errorf("failed to parse valid value from '%v' for metric %q: %w", rawValue, config.PrometheusName, err)
+					return nil, fmt.Errorf("failed to parse valid json value from '%v' for metric %q: %w", rawValue, config.PrometheusName, err)
 				}
+				labels, err := p.parseInheritedLabels(config, m, rawPayload)
+				if err != nil {
+					return nil, fmt.Errorf("failed to parse valid json labels from '%v' for metric %q: %w", rawPayload, config.PrometheusName, err)
+				}
+				m.Labels = labels
+				m.LabelsKeys = append(m.LabelsKeys, config.InheritLabels...)
 				m.Topic = topic
 				mc = append(mc, m)
 			}

pkg/metrics/parser.go

@@ -12,6 +12,7 @@ import (
 	"github.com/expr-lang/expr"
 	"github.com/expr-lang/expr/vm"
 	"github.com/hikhvar/mqtt2prometheus/pkg/config"
+	"github.com/thedevsaddam/gojsonq/v2"
 	"gopkg.in/yaml.v2"
 )
 
@@ -178,6 +179,30 @@ func (p *Parser) findMetricConfigs(metric string, deviceID string) []*config.Met
 	return configs
 }
 
+// parseInheritedLabels parses the given JSON data and extracts the listed labels
+// to append them to the Prometheus Metric
+// this function returns a map of labels
+func (p *Parser) parseInheritedLabels(cfg *config.MetricConfig, m Metric, payloadJson *gojsonq.JSONQ) (map[string]string, error) {
+
+	// includes already-defined labels that are provided by parseMetric()
+	labels := m.Labels
+
+	// inherit labels
+	if len(cfg.InheritLabels) > 0 {
+		var jsonCopy *gojsonq.JSONQ
+		for _, v := range cfg.InheritLabels {
+			jsonCopy = payloadJson.Copy()
+			result, err := jsonCopy.From(v).GetR()
+			if err != nil {
+				return labels, fmt.Errorf("failed to parse labels from '%v' for label %q: %w", jsonCopy, v, err)
+			}
+			this_label, _ := result.String()
+			labels[v] = this_label
+		}
+	}
+	return labels, nil
+}
+
 // parseMetric parses the given value according to the given deviceID and metricPath. The config allows to
 // parse a metric value according to the device ID.
 func (p *Parser) parseMetric(cfg *config.MetricConfig, metricID string, value interface{}) (Metric, error) {
@@ -209,7 +234,7 @@ func (p *Parser) parseMetric(cfg *config.MetricConfig, metricID string, value in
 				if ok {
 					metricValue = floatValue
 
-				// deprecated, replaced by ErrorValue from the upper level
+					// deprecated, replaced by ErrorValue from the upper level
 				} else if cfg.StringValueMapping.ErrorValue != nil {
 					metricValue = *cfg.StringValueMapping.ErrorValue
 				} else if cfg.ErrorValue != nil {
@@ -272,8 +297,13 @@ func (p *Parser) parseMetric(cfg *config.MetricConfig, metricID string, value in
 		ingestTime = now()
 	}
 
-	// generate dynamic labels
+	// build labels
 	var labels map[string]string
+	if len(cfg.DynamicLabels) > 0 || len(cfg.InheritLabels) > 0 {
+		labels = make(map[string]string, len(cfg.DynamicLabels)+len(cfg.InheritLabels))
+	}
+
+	// generate dynamic labels
 	if len(cfg.DynamicLabels) > 0 {
 		labels = make(map[string]string, len(cfg.DynamicLabels))
 		for k, v := range cfg.DynamicLabels {