feat: make num_elements_per_batch metric a Summary

The histogram doesn't seem to render too well on Grafana, but the
batch_size_bytes one does. This commit modifies the
num_elements_per_batch to also use a Summary with the same buckets.

.changeset/cold-worms-listen.md

+---
+'@eth-optimism/batch-submitter-service': patch
+---
+
+Switch num_elements_per_batch from Histogram to Summary

go/bss-core/metrics/metrics.go

@@ -12,11 +12,11 @@ type Metrics struct {
 	ETHBalance prometheus.Gauge
 
 	// BatchSizeInBytes tracks the size of batch submission transactions.
-	BatchSizeInBytes prometheus.Histogram
+	BatchSizeInBytes prometheus.Summary
 
 	// NumElementsPerBatch tracks the number of L2 transactions in each batch
 	// submission.
-	NumElementsPerBatch prometheus.Histogram
+	NumElementsPerBatch prometheus.Summary
 
 	// SubmissionTimestamp tracks the time at which each batch was confirmed.
 	SubmissionTimestamp prometheus.Gauge
@@ -59,23 +59,10 @@ func NewMetrics(subsystem string) *Metrics {
 			Subsystem:  subsystem,
 			Objectives: map[float64]float64{0.5: 0.05, 0.9: 0.01, 0.99: 0.001},
 		}),
-		NumElementsPerBatch: promauto.NewHistogram(prometheus.HistogramOpts{
+		NumElementsPerBatch: promauto.NewSummary(prometheus.SummaryOpts{
 			Name:       "num_elements_per_batch",
-			Help: "Number of transaction in each batch",
-			Buckets: []float64{
-				250,
-				500,
-				750,
-				1000,
-				1250,
-				1500,
-				1750,
-				2000,
-				2250,
-				2500,
-				2750,
-				3000,
-			},
+			Help:       "Number of elements in each batch",
+			Objectives: map[float64]float64{0.5: 0.05, 0.9: 0.01, 0.99: 0.001},
 			Subsystem:  subsystem,
 		}),
 		SubmissionTimestamp: promauto.NewGauge(prometheus.GaugeOpts{