Add source type label to chunk size metric (#4543)

We're tracking chunk size - but not by source type, which means we can't use the metric to locate potential chunking anomalies by source type. It's really easy to add a source type label, so I did.

pkg/engine/engine.go

@@ -832,7 +832,7 @@ func (e *Engine) scannerWorker(ctx context.Context) {
 
 		dataSize := float64(len(chunk.Data))
 
-		scanBytesPerChunk.Observe(dataSize)
+		scanBytesPerChunk.WithLabelValues(chunk.SourceType.String()).Observe(dataSize)
 		jobBytesScanned.WithLabelValues(
 			chunk.SourceType.String(),
 			chunk.SourceName,

pkg/engine/metrics.go

@@ -55,13 +55,15 @@ var (
 		[]string{"source_type", "source_name"},
 	)
 
-	scanBytesPerChunk = promauto.NewHistogram(prometheus.HistogramOpts{
+	scanBytesPerChunk = promauto.NewHistogramVec(prometheus.HistogramOpts{
 		Namespace: common.MetricsNamespace,
 		Subsystem: common.MetricsSubsystem,
 		Name:      "scan_bytes_per_chunk",
 		Help:      "Total number of bytes in a chunk.",
 		Buckets:   prometheus.ExponentialBuckets(1, 2, 18),
-	})
+	},
+		[]string{"source_type"},
+	)
 
 	jobChunksScanned = promauto.NewCounterVec(prometheus.CounterOpts{
 		Namespace: common.MetricsNamespace,