docs: Fillout metrics section of our WAL docs (#15809)

**What this PR does / why we need it**:
Add a new `metrics` section to our WAL docs.

Author: DylanGuedes

docs/sources/operations/storage/wal.md

@@ -36,6 +36,13 @@ The WAL also includes a backpressure mechanism to allow a large WAL to be replay
 
 ### Metrics
 
+The following metrics are available for monitoring the WAL:
+
+* `loki_ingester_wal_corruptions_total`: Total number of WAL corruptions encountered
+* `loki_ingester_wal_disk_full_failures_total`: Total number of disk full failures
+* `loki_ingester_wal_records_logged`: Counter for WAL records logged
+* `loki_ingester_wal_logged_bytes_total`: Total bytes written to WAL
+
 ## Changes to deployment
 
 1. Since ingesters need to have the same persistent volume across restarts/rollout, all the ingesters should be run on [StatefulSet](https://kubernetes.io/docs/concepts/workloads/controllers/statefulset/) with fixed volumes.
@@ -120,4 +127,4 @@ By following the above steps, you can ensure a smooth scaling down process for t
 
 * When the ingester restarts for any reason (upgrade, crash, etc), it should be able to attach to the same volume in order to recover back the WAL and tokens.
 * 2 ingesters should not be working with the same volume/directory for the WAL.
-* A Rollout should bring down an ingester completely and then start the new ingester, not the other way around.
+* A rollout should bring down an ingester completely and then start the new ingester, not the other way around.