doc(metric): add volume read only metric

[ChanYiLin]

ref: longhorn/longhorn#8508

Export the metrics in the following form

• metric name: longhorn_volume_file_system_read_only
• type: gauge
• if the volume becomes read-only, it has metric value 1 with the corresponding labels
  • So you know which volume is in read-only mode now
  • You can sum up the metrics to see how many volumes are in read-only mode now

# HELP longhorn_volume_file_system_read_only Volume whose mount point is in read-only mode
# TYPE longhorn_volume_file_system_read_only gauge
longhorn_volume_file_system_read_only{node="kworker1",pvc="nginx",pvc_namespace="default",volume="pvc-3a7a208b-c8c9-46a9-b2ce-4c5f740c8673"} 1

Note:
If Longhorn remounts the mount point to read-write successfully, the metrics will be cleaned up.
The metric is either 1 or no record for each volume

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[jhkrug]

lgtm