metrics: add chain/gas for cumulative gas usage

This is a followup to #31753.

A cumulative counter is more useful when we need to measure / aggregate
the metric over a longer peruid of time.
It also means we won't miss data,
e.g. our prometheus scrapes every 30 seconds,
and so may miss a transient spike in the preaggregated mgas/s.

Author: seanl-circle

core/blockchain.go

@@ -67,6 +67,7 @@ var (
 
 	chainInfoGauge   = metrics.NewRegisteredGaugeInfo("chain/info", nil)
 	chainMgaspsGauge = metrics.NewRegisteredGauge("chain/mgasps", nil)
+	chainGasCounter  = metrics.NewRegisteredCounter("chain/gas", nil)
 
 	accountReadTimer   = metrics.NewRegisteredResettingTimer("chain/account/reads", nil)
 	accountHashTimer   = metrics.NewRegisteredResettingTimer("chain/account/hashes", nil)
@@ -1844,6 +1845,7 @@ func (bc *BlockChain) insertChain(chain types.Blocks, setHead bool, makeWitness
 		// Report the import stats before returning the various results
 		stats.processed++
 		stats.usedGas += res.usedGas
+		chainGasCounter.Inc(int64(res.usedGas))
 		witness = res.witness
 
 		var snapDiffItems, snapBufItems common.StorageSize