feat: tracks block time using a Prometheus gauge metric type (#1091)

Closes #1077

I have verified the outcome of this pull request, and the block time
gauge now correctly appears in the Prometheus endpoint:

```
# HELP cometbft_consensus_block_time_seconds Duration between this block and the preceding one.
# TYPE cometbft_consensus_block_time_seconds gauge
cometbft_consensus_block_time_seconds{chain_id="mocha-4",version="1.0.0-rc16"} 11.623671263
```

consensus/metrics.go

@@ -48,6 +48,8 @@ type Metrics struct {
 
 	// Time between this and the last block.
 	BlockIntervalSeconds metrics.Histogram
+	// Block time in seconds.
+	BlockTimeSeconds metrics.Gauge
 
 	// Number of transactions.
 	NumTxs metrics.Gauge
@@ -57,9 +59,9 @@ type Metrics struct {
 	TotalTxs metrics.Gauge
 	// The latest block height.
 	CommittedHeight metrics.Gauge
-	// Whether or not a node is fast syncing. 1 if yes, 0 if no.
+	// Whether a node is fast syncing. 1 if yes, 0 if no.
 	FastSyncing metrics.Gauge
-	// Whether or not a node is state syncing. 1 if yes, 0 if no.
+	// Whether a node is state syncing. 1 if yes, 0 if no.
 	StateSyncing metrics.Gauge
 
 	// Number of blockparts transmitted by peer.
@@ -171,6 +173,12 @@ func PrometheusMetrics(namespace string, labelsAndValues ...string) *Metrics {
 			Name:      "block_interval_seconds",
 			Help:      "Time between this and the last block.",
 		}, labels).With(labelsAndValues...),
+		BlockTimeSeconds: prometheus.NewGaugeFrom(stdprometheus.GaugeOpts{
+			Namespace: namespace,
+			Subsystem: MetricsSubsystem,
+			Name:      "block_time_seconds",
+			Help:      "Duration between this block and the preceding one.",
+		}, labels).With(labelsAndValues...),
 		NumTxs: prometheus.NewGaugeFrom(stdprometheus.GaugeOpts{
 			Namespace: namespace,
 			Subsystem: MetricsSubsystem,
@@ -264,6 +272,7 @@ func NopMetrics() *Metrics {
 		ByzantineValidatorsPower: discard.NewGauge(),
 
 		BlockIntervalSeconds: discard.NewHistogram(),
+		BlockTimeSeconds:     discard.NewGauge(),
 
 		NumTxs:                    discard.NewGauge(),
 		BlockSizeBytes:            discard.NewGauge(),

consensus/state.go

@@ -1830,9 +1830,10 @@ func (cs *State) recordMetrics(height int64, block *types.Block) {
 	if height > 1 {
 		lastBlockMeta := cs.blockStore.LoadBlockMeta(height - 1)
 		if lastBlockMeta != nil {
-			cs.metrics.BlockIntervalSeconds.Observe(
-				block.Time.Sub(lastBlockMeta.Header.Time).Seconds(),
-			)
+			elapsedTime := block.Time.Sub(lastBlockMeta.Header.Time).Seconds()
+			cs.metrics.BlockIntervalSeconds.Observe(elapsedTime)
+			cs.metrics.BlockTimeSeconds.Set(elapsedTime)
+
 		}
 	}
 

test/maverick/consensus/state.go

@@ -1668,9 +1668,10 @@ func (cs *State) recordMetrics(height int64, block *types.Block) {
 	if height > 1 {
 		lastBlockMeta := cs.blockStore.LoadBlockMeta(height - 1)
 		if lastBlockMeta != nil {
-			cs.metrics.BlockIntervalSeconds.Observe(
-				block.Time.Sub(lastBlockMeta.Header.Time).Seconds(),
-			)
+			elapsedTime := block.Time.Sub(lastBlockMeta.Header.Time).Seconds()
+			cs.metrics.BlockIntervalSeconds.Observe(elapsedTime)
+			cs.metrics.BlockTimeSeconds.Set(elapsedTime)
+
 		}
 	}