Add config.storage_client documentation

This patch adds documentation for the `config.storage_client` module.
The module allows one to connect to a remote config.storage cluster and
use it as a key-value storage.

The examples on connecting to a config.storage instance via iproto has
been changed to an example on connecting to multiple config.storage
instances using the config.storage client and interacting with a
config.storage cluster.

Closes #4734

Author: georgiy-belyanin

doc/code_snippets/snippets/centralized_config/instances.enabled/config_storage/myapp.lua

@@ -1,6 +1,84 @@
-net_box = require('net.box')
-local conn = net_box.connect('127.0.0.1:4401')
 local log = require('log')
-conn:watch('config.storage:/myapp/config/all', function(key, value)
-    log.info("Configuration stored by the '/myapp/config/all' key is changed")
-end)
+config = require('config')
+
+storage_client = config.storage_client.connect({
+    {
+        uri = '127.0.0.1:4401',
+        login = 'sampleuser',
+        password = '123456',
+    },
+    {
+        uri = '127.0.0.1:4402',
+        login = 'sampleuser',
+        password = '123456',
+    },
+    {
+        uri = '127.0.0.1:4403',
+        login = 'sampleuser',
+        password = '123456',
+    },
+})
+
+function register_watchers()
+    storage_client:watch('/myapp/config/all', function(_, revision)
+        log.info("Configuration stored by the '/myapp/config/all' key is " ..
+                 "changed. New revision number is %d.", revision)
+    end)
+end
+
+register_watchers()
+
+function connect_to_configured_storage()
+    -- `config.storage.endpoints` configuration section can be
+    -- passed directly as `connect()` options to connect to the
+    -- configured config.storage cluster.
+    local endpoints = config:get('config.storage.endpoints')
+    local storage_client = config.storage_client.connect(endpoints)
+
+    return storage_client
+end
+
+function put_config()
+    local fio = require('fio')
+    local cluster_config_handle = fio.open('../../source.yaml')
+    local cluster_config = cluster_config_handle:read()
+    local response = storage_client:put('/myapp/config/all', cluster_config)
+    cluster_config_handle:close()
+
+    return response
+end
+
+function get_config_by_path()
+    local response = storage_client:get('/myapp/config/all')
+
+    return response
+end
+
+function get_config_by_prefix()
+    local response = storage_client:get('/myapp/')
+
+    return response
+end
+
+function make_txn_request()
+    -- Execute an atomic request on the config.storage cluster.
+    local response = storage_client:txn({
+        predicates = { { 'value', '==', 'v0', '/myapp/config/all' } },
+        on_success = { { 'put', '/myapp/config/all', 'v1' } },
+        on_failure = { { 'get', '/myapp/config/all' } }
+    })
+
+    return response
+end
+
+function delete_config()
+    local response = storage_client:delete('/myapp/config/all')
+
+    return response
+end
+
+function delete_all_configs()
+    local response = storage_client:delete('/')
+
+    return response
+end

doc/platform/configuration/configuration_etcd.rst

@@ -260,6 +260,8 @@ In the example below, the following options are specified:
 -   ``timeout`` specifies the interval (in seconds) to perform the status check of a configuration storage.
 -   ``reconnect_after`` specifies how much time to wait (in seconds) before reconnecting to a configuration storage.
 
+You might use :ref:`config.storage_client <config_storage_client_api_reference>` API for connecting and controlling a remote config.storage cluster.
+
 You can find the full example here: `config_storage <https://github.com/tarantool/doc/tree/latest/doc/code_snippets/snippets/centralized_config/instances.enabled/config_storage>`_.