diff --git a/pkg/prometheus/prometheus.go b/pkg/prometheus/prometheus.go
index f645065b7..3d503219f 100644
--- a/pkg/prometheus/prometheus.go
+++ b/pkg/prometheus/prometheus.go
@@ -314,6 +314,15 @@ type Data struct {
 	// This may be merged with other labels during export.
 	Labels map[string]string `json:"labels,omitempty"`
 
+	// ExternalLabels are more labels merged together with `Labels`.
+	// They can be set using SetExternalLabels.
+	// They are useful in the case where a single Data needs labels from two sources:
+	// labels specific to this data point (which should be in `Labels`), and labels
+	// that are shared between multiple data points (stored in `ExternalLabels`).
+	// This avoids allocating unique `Labels` maps for each Data struct, when
+	// most of the actual labels would be shared between them.
+	ExternalLabels map[string]string `json:"external_labels,omitempty"`
+
 	// At most one of the fields below may be set.
 	// Which one depends on the type of the metric.
 
@@ -344,6 +353,13 @@ func LabeledFloatData(metric *Metric, labels map[string]string, val float64) *Da
 	return &Data{Metric: metric, Labels: labels, Number: NewFloat(val)}
 }
 
+// SetExternalLabels sets d.ExternalLabels. See its docstring for more information.
+// Returns `d` for chainability.
+func (d *Data) SetExternalLabels(externalLabels map[string]string) *Data {
+	d.ExternalLabels = externalLabels
+	return d
+}
+
 // ExportOptions contains options that control how metric data is exported in Prometheus format.
 type ExportOptions struct {
 	// CommentHeader is prepended as a comment before any metric data is exported.
@@ -650,9 +666,9 @@ func (d *Data) writeLabelsTo(w io.Writer, extraLabels map[string]string, leLabel
 		}
 		var orderedLabels <-chan LabelOrError
 		if leLabel != nil {
-			orderedLabels = OrderedLabels(d.Labels, extraLabels, map[string]string{"le": leLabel.String()})
+			orderedLabels = OrderedLabels(d.Labels, d.ExternalLabels, extraLabels, map[string]string{"le": leLabel.String()})
 		} else {
-			orderedLabels = OrderedLabels(d.Labels, extraLabels)
+			orderedLabels = OrderedLabels(d.Labels, d.ExternalLabels, extraLabels)
 		}
 		firstLabel := true
 		var foundError error
diff --git a/runsc/cmd/metric_server.go b/runsc/cmd/metric_server.go
index 08673a69c..16337820d 100644
--- a/runsc/cmd/metric_server.go
+++ b/runsc/cmd/metric_server.go
@@ -89,11 +89,6 @@ type servedSandbox struct {
 	// Once set, it is immutable.
 	createdAt time.Time
 
-	// labelsWithMetadata is the union of `extraLabels` and `sandbox.MetricMetadata`.
-	// This is exported as the set of labels for the `sandbox_metadata` metric.
-	// Once set, it is immutable.
-	labelsWithMetadata map[string]string
-
 	// verifier allows verifying the data integrity of the metrics we get from this sandbox.
 	// It is not always initialized when the sandbox is discovered, but rather upon first metrics
 	// access to the sandbox. Metric registration data is loaded from the root container's
@@ -197,13 +192,6 @@ func (s *servedSandbox) load() (*sandbox.Sandbox, *prometheus.Verifier, error) {
 		s.verifier = verifier
 		s.cleanupVerifier = cleanup
 	}
-	s.labelsWithMetadata = make(map[string]string, len(s.extraLabels)+len(s.sandbox.MetricMetadata))
-	for k, v := range s.extraLabels {
-		s.labelsWithMetadata[k] = v
-	}
-	for k, v := range s.sandbox.MetricMetadata {
-		s.labelsWithMetadata[k] = v
-	}
 	return s.sandbox, s.verifier, nil
 }
 
@@ -691,16 +679,17 @@ func (m *MetricServer) serveMetrics(w http.ResponseWriter, req *http.Request) ht
 				func() {
 					metricsMu.Lock()
 					defer metricsMu.Unlock()
-					selfMetrics.Add(prometheus.LabeledIntData(&SandboxPresenceMetric, served.extraLabels, 1))
+					selfMetrics.Add(prometheus.LabeledIntData(&SandboxPresenceMetric, nil, 1).SetExternalLabels(served.extraLabels))
 					sandboxRunning := int64(0)
 					if isRunning {
 						sandboxRunning = 1
 						meta.numRunningSandboxes++
 					}
-					selfMetrics.Add(prometheus.LabeledIntData(&SandboxRunningMetric, served.extraLabels, sandboxRunning))
+					selfMetrics.Add(prometheus.LabeledIntData(&SandboxRunningMetric, nil, sandboxRunning).SetExternalLabels(served.extraLabels))
 					if loadErr == nil {
-						selfMetrics.Add(prometheus.LabeledIntData(&SandboxMetadataMetric, served.labelsWithMetadata, 1))
-						selfMetrics.Add(prometheus.LabeledFloatData(&SandboxCreationMetric, served.extraLabels, float64(served.createdAt.Unix())+(float64(served.createdAt.Nanosecond())/1e9)))
+						selfMetrics.Add(prometheus.LabeledIntData(&SandboxMetadataMetric, sand.MetricMetadata, 1).SetExternalLabels(served.extraLabels))
+						createdAt := float64(served.createdAt.Unix()) + (float64(served.createdAt.Nanosecond()) / 1e9)
+						selfMetrics.Add(prometheus.LabeledFloatData(&SandboxCreationMetric, nil, createdAt).SetExternalLabels(served.extraLabels))
 					}
 					if sandboxErr != nil {
 						// If the sandbox isn't running, it is normal that metrics are not exported for it, so