op-node: add clarifying comments to logBlockProcessingMetrics

Author: bitwiseguy

op-node/rollup/engine/payload_success.go

@@ -65,19 +65,25 @@ func (e *EngineController) logBlockProcessingMetrics(updateEngineFinish time.Tim
 		return
 	}
 
-	buildTime := ev.InsertStarted.Sub(ev.BuildStarted)
+	var totalTime time.Duration
+	var buildTime time.Duration
 	insertTime := updateEngineFinish.Sub(ev.InsertStarted)
 
-	var totalTime time.Duration
-	if !ev.BuildStarted.IsZero() {
-		totalTime = updateEngineFinish.Sub(ev.BuildStarted)
-	} else {
+	if ev.BuildStarted.IsZero() {
+		// BuildStarted may be zero if sequencer already built + gossiped a block, but failed during
+		// insertion and needed a retry of the insertion.
+		buildTime = 0
 		totalTime = insertTime
+	} else {
+		buildTime = ev.InsertStarted.Sub(ev.BuildStarted)
+		totalTime = updateEngineFinish.Sub(ev.BuildStarted)
 	}
 
 	// Protect against divide-by-zero
 	var mgasps float64
 	if totalTime > 0 {
+		// Calculate "block-processing" mgasps.
+		// NOTE: "realtime" mgasps (chain throughput) is a different calculation: (GasUsed / blockPeriod)
 		mgasps = float64(ev.Envelope.ExecutionPayload.GasUsed) * 1000 / float64(totalTime)
 	}