storage: mark add'l WAL latency metric essential

This change marks the `storage.wal.failover.write_and_sync.latency`
metric as "Essential" so it gets automatically pulled into the
'Essential Metrics' documentation at e.g.,
https://www.cockroachlabs.com/docs/stable/essential-metrics-self-hosted.html#storage

This is necessary since we are adding some words about this metric to
the docs via cockroachdb/docs#20566

We would like to then backport this change to all supported versions of
CockroachDB which have WAL failover (i.e., v24.1 and later).

Author: rmloveland

pkg/kv/kvserver/metrics.go

@@ -2823,6 +2823,7 @@ Note that the measurement does not include the duration for replicating the eval
 		Name:        "storage.wal.failover.write_and_sync.latency",
 		Help:        "The observed latency for writing and syncing to the logical Write-Ahead Log.",
 		Measurement: "Nanoseconds",
+		Essential:   true,
 		Unit:        metric.Unit_NANOSECONDS,
 		HowToUse: "Only populated when WAL failover is configured. Without WAL failover, the relevant " +
 			"metric is storage.wal.fsync.latency.",