metrics: change blocksize to a histogram (#6549)

## Description

Change block_size gauge to a histogram to observe block size overtime

This will help will see which chains have full blocks vs empty. 

closes #5752

CHANGELOG_PENDING.md

@@ -118,6 +118,7 @@ Friendly reminder: We have a [bug bounty program](https://hackerone.com/tendermi
 - [crypto/merkle] \#6443 Improve HashAlternatives performance (@cuonglm)
 - [crypto/merkle] \#6513 Optimize HashAlternatives (@marbar3778)
 - [p2p/pex] \#6509 Improve addrBook.hash performance (@cuonglm)
+- [consensus/metrics] \#6549 Change block_size gauge to a histogram for better observability over time (@marbar3778)
 
 ### BUG FIXES
 

internal/consensus/metrics.go

@@ -48,7 +48,7 @@ type Metrics struct {
 	// Number of transactions.
 	NumTxs metrics.Gauge
 	// Size of the block.
-	BlockSizeBytes metrics.Gauge
+	BlockSizeBytes metrics.Histogram
 	// Total number of transactions.
 	TotalTxs metrics.Gauge
 	// The latest block height.
@@ -150,7 +150,7 @@ func PrometheusMetrics(namespace string, labelsAndValues ...string) *Metrics {
 			Name:      "num_txs",
 			Help:      "Number of transactions.",
 		}, labels).With(labelsAndValues...),
-		BlockSizeBytes: prometheus.NewGaugeFrom(stdprometheus.GaugeOpts{
+		BlockSizeBytes: prometheus.NewHistogramFrom(stdprometheus.HistogramOpts{
 			Namespace: namespace,
 			Subsystem: MetricsSubsystem,
 			Name:      "block_size_bytes",
@@ -210,7 +210,7 @@ func NopMetrics() *Metrics {
 		BlockIntervalSeconds: discard.NewHistogram(),
 
 		NumTxs:          discard.NewGauge(),
-		BlockSizeBytes:  discard.NewGauge(),
+		BlockSizeBytes:  discard.NewHistogram(),
 		TotalTxs:        discard.NewGauge(),
 		CommittedHeight: discard.NewGauge(),
 		FastSyncing:     discard.NewGauge(),

internal/consensus/state.go

@@ -1809,7 +1809,7 @@ func (cs *State) recordMetrics(height int64, block *types.Block) {
 
 	cs.metrics.NumTxs.Set(float64(len(block.Data.Txs)))
 	cs.metrics.TotalTxs.Add(float64(len(block.Data.Txs)))
-	cs.metrics.BlockSizeBytes.Set(float64(block.Size()))
+	cs.metrics.BlockSizeBytes.Observe(float64(block.Size()))
 	cs.metrics.CommittedHeight.Set(float64(block.Height))
 }