core: log message when reloading finishes

Reloading might be slow, especially when under memory pressure, and watchdogs
might be triggered. It is useful to have timestamped telemetry in the journal
to see how long a reload takes.

src/core/main.c

@@ -1973,7 +1973,7 @@ static int invoke_main_loop(
 
                         manager_send_reloading(m);
 
-                        log_info("Reloading.");
+                        log_info("Reloading...");
 
                         /* First, save any overridden log level/target, then parse the configuration file,
                          * which might change the log level to new settings. */
@@ -1998,6 +1998,9 @@ static int invoke_main_loop(
                                 /* Reloading failed before the point of no return.
                                  * Let's continue running as if nothing happened. */
                                 m->objective = MANAGER_OK;
+                        else
+                                log_info("Reloading finished in " USEC_FMT " ms.",
+                                         usec_sub_unsigned(now(CLOCK_MONOTONIC), m->timestamps[MANAGER_TIMESTAMP_UNITS_LOAD].monotonic) / USEC_PER_MSEC);
 
                         continue;
                 }