Fix missing swap fields in linux/memory

CHANGELOG.next.asciidoc

@@ -67,6 +67,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...main[Check the HEAD dif
 - Enhance metricbeat on openshift documentation {pull}30054[30054]
 - Fixed missing ZooKeeper metrics due compatibility issues with versions >= 3.6.0 {pull}30068[30068]
 - Fix Docker module: rename fields on dashboards. {pull}30500[30500]
+- Add back missing metrics to system/linux. {pull}30774[30774]
 
 *Packetbeat*
 

metricbeat/docs/fields.asciidoc

@@ -46458,6 +46458,100 @@ format: bytes
 
 --
 
+[float]
+=== swap
+
+This group contains statistics related to the swap memory usage on the system.
+
+
+*`linux.memory.swap.total`*::
++
+--
+Total swap memory.
+
+
+type: long
+
+format: bytes
+
+--
+
+*`linux.memory.swap.used.bytes`*::
++
+--
+Used swap memory.
+
+
+type: long
+
+format: bytes
+
+--
+
+*`linux.memory.swap.free`*::
++
+--
+Available swap memory.
+
+
+type: long
+
+format: bytes
+
+--
+
+*`linux.memory.swap.used.pct`*::
++
+--
+The percentage of used swap memory.
+
+
+type: scaled_float
+
+format: percent
+
+--
+
+*`linux.memory.swap.in.pages`*::
++
+--
+Pages swapped in.
+
+
+type: long
+
+--
+
+*`linux.memory.swap.out.pages`*::
++
+--
+Pages swapped out.
+
+
+type: long
+
+--
+
+*`linux.memory.swap.readahead.cached`*::
++
+--
+Swap readahead pages hit from swap_ra_hit.
+
+
+type: long
+
+--
+
+*`linux.memory.swap.readahead.pages`*::
++
+--
+Pages swapped based on readahead predictions.
+
+
+type: long
+
+--
+
 [float]
 === pageinfo