ra_aux: Expose current_term

With `current_term/1` (this change), `last_applied/1` and
`effective_machine_version/1` from the parent commit, the aux API
reflects essentially the same information as is passed as metadata to
the `RaMachine:apply/3` callback (`ra_machine:command_meta_data()`).

src/ra_aux.erl

@@ -10,6 +10,7 @@
          effective_machine_version/1,
          leader_id/1,
          last_applied/1,
+         current_term/1,
          members_info/1,
          overview/1,
          log_last_index_term/1,
@@ -41,6 +42,10 @@ leader_id(State) ->
 last_applied(State) ->
     maps:get(?FUNCTION_NAME, State).
 
+-spec current_term(ra_aux:internal_state()) -> ra_term().
+current_term(State) ->
+    maps:get(?FUNCTION_NAME, State).
+
 -spec members_info(ra_aux:internal_state()) -> ra_cluster().
 members_info(State) ->
     ra_server:state_query(?FUNCTION_NAME, State).