add support for monitoring LVM caches (Closes: prometheus-community#158)

This is basically a copy-paste of the snapshots code, and seems to
work on my end.

lvm-prom-collector

@@ -48,14 +48,17 @@ snapshots=false
 physical=false
 groups=false
 
-while getopts "htpsg" opt; do
+while getopts "htpscg" opt; do
   case $opt in
     p)
       physical=true
       ;;
     s)
       snapshots=true
       ;;
+    c)
+      caches=true
+      ;;
     g)
       groups=true
       ;;
@@ -105,6 +108,20 @@ if [ "$snapshots" = true ]; then
   done
 fi
 
+if [ "$caches" = true ]; then
+  echo "# HELP node_lvm_caches_allocated percentage of allocated data to a cache"
+  echo "# TYPE node_lvm_caches_allocated gauge"
+
+  lvs_output=$(lvs --noheadings --select 'lv_attr=~[^C.*]' --units b --nosuffix --unquoted --nameprefixes --options lv_uuid,vg_name,data_percent 2>/dev/null)
+  echo "$lvs_output" | while IFS= read -r line; do
+    # Skip if the line is empty
+    [ -z "$line" ] && continue
+    # shellcheck disable=SC2086
+    declare $line
+    echo "node_lvm_caches_allocated{uuid=\"$LVM2_LV_UUID\", vgroup=\"$LVM2_VG_NAME\"} $LVM2_DATA_PERCENT"
+  done
+fi
+
 if [ "$thin_pools" = true ]; then
   lvs_output=$(lvs --noheadings --select 'lv_attr=~[^t.*]' --units b --nosuffix --unquoted --nameprefixes --options lv_uuid,vg_name,data_percent,metadata_percent 2>/dev/null)