Add new file

doc/api/stat-lease4-get.json

+{
+    "name": "stat-lease4-get",
+    "brief": "The stat-lease4-get command fetches lease statistics for a range of known subnets.",
+    "description": "See <xref linkend=\"idp81\"/>",
+    "support": [ "kea-dhcp4", "kea-dhcp6" ],
+    "avail": "1.4.0",
+    "hook": "stat_cmds",
+    "cmd-syntax": "{
+  \"command\": \"stat-lease4-get\"
+}",
+    "resp-syntax": "{
+    \"result\": 0,
+    \"text\": \"stat-lease4-get: 2 rows found\",
+    \"arguments\": {
+      \"result-set\": {
+        \"columns\": [ \"subnet-id\", \"total-addresses\", \"assigned-addresses\", \"declined-addresses\" ]
+        \"rows\": [
+          [ 10, 256, 111, 0 ],
+          [ 20, 4098, 2034, 4 ]
+        ],
+      \"timestamp\": \"2018-05-04 15:03:37.000000\"
+      }
+    }
+  }",
+}
\ No newline at end of file