Add counter filter (#3921)

* Add counter filter

* move counter filter doc to existing chapter

* Use existing typerror exception from Counter

* Match counter filter example task name and output

docs/docsite/rst/filter_guide.rst

@@ -297,6 +297,84 @@ This produces:
 
 .. versionadded: 2.0.0
 
+Counting elements in a sequence
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The ``community.general.counter`` filter plugin allows you to count (hashable) elements in a sequence. Elements are returned as dictionary keys and their counts are stored as dictionary values.
+
+.. code-block:: yaml+jinja
+
+    - name: Count character occurrences in a string
+      debug:
+        msg: "{{ 'abccbaabca' | community.general.counter }}"
+
+    - name: Count items in a list
+      debug:
+        msg: "{{ ['car', 'car', 'bike', 'plane', 'bike'] | community.general.counter }}"
+
+This produces:
+
+.. code-block:: ansible-output
+
+    TASK [Count character occurrences in a string] ********************************************
+    ok: [localhost] => {
+        "msg": {
+            "a": 4,
+            "b": 3,
+            "c": 3
+        }
+    }
+
+    TASK [Count items in a list] **************************************************************
+    ok: [localhost] => {
+        "msg": {
+            "bike": 2,
+            "car": 2,
+            "plane": 1
+        }
+    }
+
+This plugin is useful for selecting resources based on current allocation:
+
+.. code-block:: yaml+jinja
+
+    - name: Get ID of SCSI controller(s) with less than 4 disks attached and choose the one with the least disks
+      debug:
+        msg: >-
+          {{
+             ( disks | dict2items | map(attribute='value.adapter') | list
+               | community.general.counter | dict2items
+               | rejectattr('value', '>=', 4) | sort(attribute='value') | first
+             ).key
+          }}
+      vars:
+        disks:
+          sda:
+            adapter: scsi_1
+          sdb:
+            adapter: scsi_1
+          sdc:
+            adapter: scsi_1
+          sdd:
+            adapter: scsi_1
+          sde:
+            adapter: scsi_2
+          sdf:
+            adapter: scsi_3
+          sdg:
+            adapter: scsi_3
+
+This produces:
+
+.. code-block:: ansible-output
+
+    TASK [Get ID of SCSI controller(s) with less than 4 disks attached and choose the one with the least disks]
+    ok: [localhost] => {
+        "msg": "scsi_2"
+    }
+
+.. versionadded:: 4.3.0
+
 Working with times
 ------------------