box: add "leader_name" field to the box.info.election

Prior to this patch, the table had no information about the leader
other than his id in the "leader" field. It may not be convenient for
the user to search for a name corresponding to a given id. Much more
convenient to see the leader's name in box.info.election.

Closes #8931

@TarantoolBot document
Title: Document `box.info.election`

box.info.election now contains one more field: `leader_name`: string.
There are several possible values ​​for this field:

 - `nil`, if there is no leader in a cluster.

 - `box.NULL`, if there is a leader, but he does not have a name.

 - `some string`, if there is a leader and he has a name.

Example:

```console
tarantool> box.info.election
---
- leader_idle: 0
  leader_name: node1
  state: leader
  vote: 1
  term: 3
  leader: 1
...
```

[box-info-election] https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_info/election/

changelogs/unreleased/gh-8931-box-info-election-leader-name.md

+## feature/box
+
+* Added the `leader_name` field to `box.info.election` (gh-8931).

src/box/lua/info.c

@@ -676,6 +676,14 @@ lbox_info_election(struct lua_State *L)
 	lua_pushinteger(L, raft->leader);
 	lua_setfield(L, -2, "leader");
 	if (raft_is_enabled(raft)) {
+		if (raft->leader != 0) {
+			char *leader_name = replica_by_id(raft->leader)->name;
+			if (*leader_name == 0)
+				luaL_pushnull(L);
+			else
+				lua_pushstring(L, leader_name);
+			lua_setfield(L, -2, "leader_name");
+		}
 		lua_pushnumber(L, raft_leader_idle(raft));
 		lua_setfield(L, -2, "leader_idle");
 	}

test/replication-luatest/gh_8931_box_info_election_leader_name_test.lua

+local t = require('luatest')
+local cluster = require('luatest.replica_set')
+local server = require('luatest.server')
+
+local g = t.group('gh-8931')
+--
+-- gh-8931:
+-- There is the leader field in the given informational API, but it is easier
+-- to a human to understand human-readable names. We have instance names
+-- since #5029. Let's show them in this API.
+--
+g.before_all(function(cg)
+    cg.cluster = cluster:new({})
+    local cfg = {
+        replication = {
+            server.build_listen_uri('node1', cg.cluster.id),
+            server.build_listen_uri('node2', cg.cluster.id)
+        },
+        election_mode = 'candidate'
+    }
+    cg.node1 = cg.cluster:build_and_add_server({alias = 'node1', box_cfg = cfg})
+    cg.node2 = cg.cluster:build_and_add_server({alias = 'node2', box_cfg = cfg})
+    cg.cluster:start()
+end)
+
+g.before_test('test_leader_has_no_name', function(cg)
+    cg.node1:wait_until_election_leader_found()
+    cg.node2:wait_until_election_leader_found()
+end)
+
+g.test_leader_has_no_name = function(cg)
+    cg.node1:exec(function()
+        local function is_null(val)
+            if val == nil and val then
+                return true
+            end
+            return false
+        end
+        t.assert(is_null(box.info.election.leader_name))
+    end)
+end
+
+g.before_test('test_leader_has_a_name', function(cg)
+    cg.node1:exec(function()
+        box.cfg{instance_name = 'node1'}
+    end)
+    cg.node2:exec(function()
+        box.cfg{instance_name = 'node2'}
+    end)
+end)
+
+g.test_leader_has_a_name = function(cg)
+    cg.cluster:get_leader():exec(function()
+        t.assert_equals(box.info.election.leader_name, box.info.name)
+    end)
+end
+
+g.before_test('test_no_leader', function(cg)
+    cg.node1:exec(function()
+        box.cfg{election_mode = 'off'}
+    end)
+    cg.node2:exec(function()
+        box.cfg{election_mode = 'off'}
+    end)
+    cg.node1:wait_for_election_state('follower')
+    cg.node2:wait_for_election_state('follower')
+end)
+
+g.test_no_leader = function(cg)
+    cg.node1:exec(function()
+        local function is_nil(val)
+            if val == nil and not val then
+                return true
+            end
+            return false
+        end
+        t.assert(is_nil(box.info.election.leader_name))
+    end)
+end
+
+g.after_all(function(cg)
+    cg.cluster:drop()
+end)