Add metrics exposing extended md RAID info (#958)

Add metrics that expose more information about MD RAID devices and
disks:

- the RAID level in use
- the RAID set that a disk belongs to

This allows for things like alert on unusually high I/O
utilisation for a disk compared to other disks in the same RAID set,
which usually means the disk is failing, and for comparing
write/read latency across RAID sets.

Output looks like:

    node_md_disk_info{disk_device="/dev/dm-0", md_device="md1", md_set="A"} 1
    node_md_disk_info{disk_device="/dev/dm-3", md_device="md1", md_set="B"} 1
    node_md_disk_info{disk_device="/dev/dm-2", md_device="md1", md_set="A"} 1
    node_md_disk_info{disk_device="/dev/dm-1", md_device="md1", md_set="B"} 1
    node_md_disk_info{disk_device="/dev/dm-4", md_device="md1", md_set="A"} 1
    node_md_disk_info{disk_device="/dev/dm-5", md_device="md1", md_set="B"} 1
    node_md_info{md_device="md1", md_name="foo", raid_level="10", md_metadata_version="1.2"} 1

The `node_md_info` metric, which gives additional information about the
RAID array, is intentionally separate to avoid adding all of those
labels to each disk. If you need to query using the labels contained in
`node_md_info`, you can do that using PromQL:
https://www.robustperception.io/how-to-have-labels-for-machine-roles/

I looked at adding the array UUID, but there's no sysfs entry for it and
I'm not sure there's a strong use case for it.

This patch to add a sysfs entry for the UUID was apparently not
accepted:
https://www.spinics.net/lists/raid/msg40667.html

Add these metrics as a textfile script rather than adding them to the Go
'md' module as they're perhaps less commonly useful. If lots of people
find them useful, we can later rewrite this in Go.

Signed-off-by: Matt Bostock <mbostock@cloudflare.com>

text_collector_examples/md_info.sh

@@ -0,0 +1,56 @@
+#!/usr/bin/env bash
+set -eu
+
+for MD_DEVICE in /dev/md/*; do
+  # Subshell to avoid eval'd variables from leaking between iterations
+  (
+    # Resolve symlink to discover device, e.g. /dev/md127
+    MD_DEVICE_NUM=$(readlink -f "${MD_DEVICE}")
+
+    # Remove /dev/ prefix
+    MD_DEVICE_NUM=${MD_DEVICE_NUM#/dev/}
+    MD_DEVICE=${MD_DEVICE#/dev/md/}
+
+    # Query sysfs for info about md device
+    SYSFS_BASE="/sys/devices/virtual/block/${MD_DEVICE_NUM}/md"
+    MD_LAYOUT=$(cat "${SYSFS_BASE}/layout")
+    MD_LEVEL=$(cat "${SYSFS_BASE}/level")
+    MD_METADATA_VERSION=$(cat "${SYSFS_BASE}/metadata_version")
+    MD_NUM_RAID_DISKS=$(cat "${SYSFS_BASE}/raid_disks")
+
+    # Remove 'raid' prefix from RAID level
+    MD_LEVEL=${MD_LEVEL#raid}
+
+    # Output disk metrics
+    for RAID_DISK in ${SYSFS_BASE}/rd[0-9]*; do
+      DISK=$(readlink -f "${RAID_DISK}/block")
+      DISK_DEVICE=$(basename "${DISK}")
+      RAID_DISK_DEVICE=$(basename "${RAID_DISK}")
+      RAID_DISK_INDEX=${RAID_DISK_DEVICE#rd}
+      RAID_DISK_STATE=$(cat "${RAID_DISK}/state")
+
+      DISK_SET=""
+      # Determine disk set using logic from mdadm: https://github.com/neilbrown/mdadm/commit/2c096ebe4b
+      if [[ ${RAID_DISK_STATE} == "in_sync" && ${MD_LEVEL} == 10 && $((MD_LAYOUT & ~0x1ffff)) ]]; then
+        NEAR_COPIES=$((MD_LAYOUT & 0xff))
+        FAR_COPIES=$(((MD_LAYOUT >> 8) & 0xff))
+        COPIES=$((NEAR_COPIES * FAR_COPIES))
+
+        if [[ $((MD_NUM_RAID_DISKS % COPIES == 0)) && $((COPIES <= 26)) ]]; then
+          DISK_SET=$((RAID_DISK_INDEX % COPIES))
+        fi
+      fi
+
+      echo -n "node_md_disk_info{disk_device=\"${DISK_DEVICE}\", md_device=\"${MD_DEVICE_NUM}\""
+      if [[ -n ${DISK_SET} ]]; then
+        SET_LETTERS=({A..Z})
+        echo -n ", md_set=\"${SET_LETTERS[${DISK_SET}]}\""
+      fi
+      echo "} 1"
+    done
+
+    # Output RAID array metrics
+    # NOTE: Metadata version is a label rather than a separate metric because the version can be a string
+    echo "node_md_info{md_device=\"${MD_DEVICE_NUM}\", md_name=\"${MD_DEVICE}\", raid_level=\"${MD_LEVEL}\", md_metadata_version=\"${MD_METADATA_VERSION}\"} 1"
+  )
+done