change consensus_block_interval_seconds metric type to gauge

Otherwise, it's impossible to see outliers
https://github.com/tendermint/tendermint/issues/1835#issuecomment-402054099

CHANGELOG_PENDING.md

@@ -21,5 +21,6 @@ FEATURES:
 
 IMPROVEMENTS:
 - [types] add Address to GenesisValidator [\#1714](https://github.com/tendermint/tendermint/issues/1714)
+- [metrics] `consensus.block_interval_metrics` is now gauge, not histogram (you will be able to see spikes, if any)
 
 BUG FIXES:

consensus/metrics.go

@@ -30,7 +30,7 @@ type Metrics struct {
 	ByzantineValidatorsPower metrics.Gauge
 
 	// Time between this and the last block.
-	BlockIntervalSeconds metrics.Histogram
+	BlockIntervalSeconds metrics.Gauge
 
 	// Number of transactions.
 	NumTxs metrics.Gauge
@@ -85,11 +85,11 @@ func PrometheusMetrics() *Metrics {
 			Help:      "Total power of the byzantine validators.",
 		}, []string{}),
 
-		BlockIntervalSeconds: prometheus.NewHistogramFrom(stdprometheus.HistogramOpts{
+
+		BlockIntervalSeconds: prometheus.NewGaugeFrom(stdprometheus.GaugeOpts{
 			Subsystem: "consensus",
 			Name:      "block_interval_seconds",
 			Help:      "Time between this and the last block.",
-			Buckets:   []float64{1, 2.5, 5, 10, 60},
 		}, []string{}),
 
 		NumTxs: prometheus.NewGaugeFrom(stdprometheus.GaugeOpts{
@@ -124,7 +124,7 @@ func NopMetrics() *Metrics {
 		ByzantineValidators:      discard.NewGauge(),
 		ByzantineValidatorsPower: discard.NewGauge(),
 
-		BlockIntervalSeconds: discard.NewHistogram(),
+		BlockIntervalSeconds: discard.NewGauge(),
 
 		NumTxs:         discard.NewGauge(),
 		BlockSizeBytes: discard.NewGauge(),

consensus/state.go

@@ -1374,7 +1374,7 @@ func (cs *ConsensusState) recordMetrics(height int64, block *types.Block) {
 
 	if height > 1 {
 		lastBlockMeta := cs.blockStore.LoadBlockMeta(height - 1)
-		cs.metrics.BlockIntervalSeconds.Observe(
+		cs.metrics.BlockIntervalSeconds.Set(
 			block.Time.Sub(lastBlockMeta.Header.Time).Seconds(),
 		)
 	}