Prometheus Metrics: Turn notifications_total counter into notifications_sizes histogram (#5535)

* Turn notifications_total into notifications_sizes

* Address review

substrate/utils/prometheus/src/lib.rs

@@ -19,6 +19,7 @@ pub use prometheus::{
 	self,
 	Registry, Error as PrometheusError, Opts,
 	Histogram, HistogramOpts, HistogramVec,
+	exponential_buckets,
 	core::{
 		GenericGauge as Gauge, GenericCounter as Counter,
 		GenericGaugeVec as GaugeVec, GenericCounterVec as CounterVec,