raft, api: Add RESTful API to query current leader of a raft group

Example:

  $ curl -X GET "http://127.0.0.1:10000/raft/leader_host"
  "f7f57588-62de-4cac-9e4b-c62bfc458d91"

Accepts optional group_id param, defaults to group0.

api/api-doc/raft.json

@@ -38,6 +38,30 @@
                ]
             }
          ]
+      },
+      {
+         "path":"/raft/leader_host",
+         "operations":[
+            {
+               "method":"GET",
+               "summary":"Returns host ID of the current leader of the given Raft group",
+               "type":"string",
+               "nickname":"get_leader_host",
+               "produces":[
+                  "application/json"
+               ],
+               "parameters":[
+                  {
+                     "name":"group_id",
+                     "description":"The ID of the group. When absent, group0 is used.",
+                     "required":false,
+                     "allowMultiple":false,
+                     "type":"string",
+                     "paramType":"query"
+                  }
+               ]
+            }
+         ]
       }
    ]
 }

api/raft.cc

@@ -60,10 +60,24 @@ void set_raft(http_context&, httpd::routes& r, sharded<service::raft_group_regis
 
         co_return json_void{};
     });
+    r::get_leader_host.set(r, [&raft_gr] (std::unique_ptr<http::request> req) -> future<json_return_type> {
+        return smp::submit_to(0, [&] {
+            auto& srv = std::invoke([&] () -> raft::server& {
+                if (req->query_parameters.contains("group_id")) {
+                    raft::group_id id{utils::UUID{req->get_query_param("group_id")}};
+                    return raft_gr.local().get_server(id);
+                } else {
+                    return raft_gr.local().group0();
+                }
+            });
+            return json_return_type(srv.current_leader().to_sstring());
+        });
+    });
 }
 
 void unset_raft(http_context&, httpd::routes& r) {
     r::trigger_snapshot.unset(r);
+    r::get_leader_host.unset(r);
 }
 
 }