ts: record child metrics with low frequency poller

This change adds the ability to get changefeed child metrics to be recorded. The child metrics have their names augmented with their labels.

Epic: CRDB-55079
Release: None

Author: jasonlmfong

pkg/server/status/recorder.go

@@ -14,7 +14,9 @@ import (
 	"math"
 	"os"
 	"runtime"
+	"sort"
 	"strconv"
+	"strings"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -455,7 +457,42 @@ func (mr *MetricsRecorder) GetTimeSeriesData(childMetrics bool) []tspb.TimeSerie
 		if !ChildMetricsStorageEnabled.Get(&mr.settings.SV) {
 			return nil
 		}
-		return nil // TODO(jasonlmfong): to be implemented
+
+		mr.mu.RLock()
+		defer mr.mu.RUnlock()
+
+		if mr.mu.nodeRegistry == nil {
+			// We haven't yet processed initialization information; do nothing.
+			if log.V(1) {
+				log.Dev.Warning(context.TODO(), "MetricsRecorder.GetChildMetricsData() called before NodeID allocation")
+			}
+			return nil
+		}
+
+		data := make([]tspb.TimeSeriesData, 0)
+
+		// Record child metrics from app registry for system tenant only.
+		now := mr.clock.Now()
+		recorder := registryRecorder{
+			registry:       mr.mu.appRegistry,
+			format:         nodeTimeSeriesPrefix,
+			source:         mr.mu.desc.NodeID.String(),
+			timestampNanos: now.UnixNano(),
+		}
+		recorder.recordChangefeedChildMetrics(&data)
+
+		// Record child metrics from app-level registries for secondary tenants
+		for tenantID, r := range mr.mu.tenantRegistries {
+			tenantRecorder := registryRecorder{
+				registry:       r,
+				format:         nodeTimeSeriesPrefix,
+				source:         tsutil.MakeTenantSource(mr.mu.desc.NodeID.String(), tenantID.String()),
+				timestampNanos: now.UnixNano(),
+			}
+			tenantRecorder.recordChangefeedChildMetrics(&data)
+		}
+
+		return data
 	}
 
 	lastDataCount := atomic.LoadInt64(&mr.lastDataCount)
@@ -906,6 +943,93 @@ func (rr registryRecorder) recordChild(
 	})
 }
 
+// recordChangefeedChildMetrics iterates through changefeed metrics in the registry and processes child metrics
+// for those that have EnableLowFreqChildCollection set to true in their metadata.
+// Records up to 1024 child metrics to prevent unbounded memory usage and performance issues.
+//
+// NB: Only available for Counter and Gauge metrics.
+func (rr registryRecorder) recordChangefeedChildMetrics(dest *[]tspb.TimeSeriesData) {
+	maxChildMetrics := 1024
+
+	var childMetricsCount int
+	labels := rr.registry.GetLabels()
+	rr.registry.Each(func(name string, v interface{}) {
+		// Filter for changefeed metrics only
+		if !strings.HasPrefix(name, "changefeed.") {
+			return
+		}
+
+		prom, ok := v.(metric.PrometheusExportable)
+		if !ok {
+			return
+		}
+		promIter, ok := v.(metric.PrometheusIterable)
+		if !ok {
+			return
+		}
+
+		// Check if the metric has child collection enabled in its metadata
+		if iterable, ok := v.(metric.Iterable); ok {
+			metadata := iterable.GetMetadata()
+			if !metadata.GetEnableLowFreqChildCollection() {
+				return // Skip this metric if child collection is not enabled
+			}
+		} else {
+			// If we can't get metadata, skip child collection for safety
+			return
+		}
+		m := prom.ToPrometheusMetric()
+		m.Label = append(labels, prom.GetLabels(false /* useStaticLabels */)...)
+
+		processChildMetric := func(childMetric *prometheusgo.Metric) {
+			if childMetricsCount >= maxChildMetrics {
+				return // Stop processing once we hit the limit
+			}
+
+			// Sanitize and sort labels using the same sanitization as Prometheus export
+			sanitizedLabels := make([]struct{ key, value string }, len(childMetric.Label))
+			for i, label := range childMetric.Label {
+				sanitizedLabels[i].key = metric.ExportedLabel(label.GetName())
+				sanitizedLabels[i].value = label.GetValue()
+			}
+			sort.Slice(sanitizedLabels, func(i, j int) bool {
+				return sanitizedLabels[i].key < sanitizedLabels[j].key
+			})
+
+			// Build labels suffix using sanitized and sorted key-value pairs
+			var labelsSuffix string
+			for _, label := range sanitizedLabels {
+				labelsSuffix += fmt.Sprintf("%s=\"%s\",", label.key, label.value)
+			}
+			// Remove trailing comma and wrap with curly braces
+			if len(labelsSuffix) > 0 {
+				labelsSuffix = "{" + labelsSuffix[:len(labelsSuffix)-1] + "}"
+			}
+
+			var value float64
+			if childMetric.Gauge != nil {
+				value = *childMetric.Gauge.Value
+			} else if childMetric.Counter != nil {
+				value = *childMetric.Counter.Value
+			} else {
+				return
+			}
+			*dest = append(*dest, tspb.TimeSeriesData{
+				Name:   fmt.Sprintf(rr.format, prom.GetName(false /* useStaticLabels */)+labelsSuffix),
+				Source: rr.source,
+				Datapoints: []tspb.TimeSeriesDatapoint{
+					{
+						TimestampNanos: rr.timestampNanos,
+						Value:          value,
+					},
+				},
+			})
+			childMetricsCount++
+		}
+		promIter.Each(m.Label, processChildMetric)
+	})
+}
+
 // GetTotalMemory returns either the total system memory (in bytes) or if
 // possible the cgroups available memory.
 func GetTotalMemory(ctx context.Context) (int64, error) {