doc: use extended format in custom plugin build

Advice users to iterate through extensive
metrics.collect{extended_format = true} output instead of collectors.

Part of tarantool/tarantool#7725
Part of tarantool/tarantool#7728

doc/monitoring/plugins.rst

@@ -260,17 +260,23 @@ Include the following in your main export function:
 
 ..  code-block:: lua
 
-    -- Invoke all callbacks registered via `metrics.register_callback(<callback-function>)`
-    metrics.invoke_callbacks()
+    local metrics = require('metrics')
+    local string_utils = require('metrics.string_utils')
 
-    -- Loop over collectors
-    for _, c in pairs(metrics.collectors()) do
-        ...
+    -- Collect up-to-date metrics with extended format.
+    local output = metrics.collect{invoke_callbacks = true, extended_format = true}
 
-        -- Loop over instant observations in the collector
-        for _, obs in pairs(c:collect()) do
-            -- Export observation `obs`
-            ...
-        end
+    for _, coll_obs in pairs(output) do
+        -- Serialize collector info like coll_obs.name, coll_obs.help,
+        -- coll_obs.kind and coll_obs.timestamp
+
+        for group_name, obs_group in pairs(coll_obs.observations) do
+            -- Common way to build metric name.
+            local metric_name = string_utils.build_name(coll_obs.name, group_name)
+
+            for _, obs in pairs(obs_group) do
+                -- Serialize observation info: obs.value and obs.label_pairs
 
+            end
+        end
     end