add timestamp to mailed journal log.

Also restrict to 200 rows to prevent unreasonable large emails...

src/sysalert/email.py

@@ -50,7 +50,11 @@ def failure(name, failures, config):
     reader.add_match(MONITOR_INVOCATION_ID=latest_inv_id)
     reader.add_disjunction()
     reader.add_match(_SYSTEMD_INVOCATION_ID=latest_inv_id)
-    journal_txt = "\n".join([entry['MESSAGE'] for entry in reader])
+    journal_txt = "\n".join(
+            [
+                f'{entry["_SOURCE_REALTIME_TIMESTAMP"]}: {entry["MESSAGE"]}'
+                for entry in list(reader)[-200:]
+            ])
 
     if nr_failures <= 1:
         subject=f"{hostname} - {name}: failure"