Add example for gears.table.count_keys

Signed-off-by: Lucas Schwiderski <lucas@lschwiderski.de>

lib/gears/table.lua

@@ -169,6 +169,8 @@ end
 --
 -- This is functionally equivalent, but faster than `#gears.table.keys(t)`.
 --
+-- @DOC_text_gears_table_count_keys_EXAMPLE@
+--
 -- @tparam table t The table for which to count the keys.
 -- @treturn number The number of keys in the table.
 -- @staticfct gears.table.count_keys

tests/examples/text/gears/table/count_keys.lua

@@ -0,0 +1,6 @@
+--DOC_GEN_OUTPUT --DOC_HIDE
+local gears = require("gears") --DOC_HIDE
+
+local tab = { 1, nil, "a", "b", foo = "bar" }
+local count = gears.table.count_keys(tab)
+print("The table has " .. count .. " keys")