Add progress log to alert packaging loop.

The log ensures that DiaPipelineTask does not appear to "lock up" while
processing alerts. This is motivated in part by Prompt Processing,
which assumes that any active pod emits at least one log every
30 seconds.

The logging overhead will naturally decrease as the task speeds up; run
times shorter than 10 seconds will log only the basic milestones.

python/lsst/ap/association/packageAlerts.py

@@ -22,6 +22,7 @@
 __all__ = ("PackageAlertsConfig", "PackageAlertsTask")
 
 import io
+import logging
 import os
 import sys
 import time
@@ -48,6 +49,7 @@
 import lsst.pex.config as pexConfig
 from lsst.pex.exceptions import InvalidParameterError
 import lsst.pipe.base as pipeBase
+import lsst.utils.logging
 from lsst.utils.timer import timeMethod
 
 
@@ -247,7 +249,14 @@ def run(self,
         diffImPhotoCalib = diffIm.getPhotoCalib()
         calexpPhotoCalib = calexp.getPhotoCalib()
         templatePhotoCalib = template.getPhotoCalib()
+
+        n_sources = len(diaSourceCat)
+        self.log.info("Packaging alerts for %d DiaSources.", n_sources)
+        # Log every 10 seconds as proof of liveness.
+        loop_logger = lsst.utils.logging.PeriodicLogger(self.log, interval=10.0, level=logging.DEBUG)
+
         for srcIndex, diaSource in diaSourceCat.iterrows():
+            loop_logger.log("%s/%s sources have been packaged.", len(alerts), n_sources)
             # Get all diaSources for the associated diaObject.
             # TODO: DM-31992 skip DiaSources associated with Solar System
             #       Objects for now.
@@ -305,10 +314,13 @@ def run(self,
                                    templateCutout))
 
         if self.config.doProduceAlerts:
+            self.log.info("Producing alerts to %s.", self.kafkaTopic)
             self.produceAlerts(alerts, visit, detector)
 
         if self.config.doWriteAlerts:
-            with open(os.path.join(self.config.alertWriteLocation, f"{visit}_{detector}.avro"), "wb") as f:
+            avro_path = os.path.join(self.config.alertWriteLocation, f"{visit}_{detector}.avro")
+            self.log.info("Writing alerts to %s.", avro_path)
+            with open(avro_path, "wb") as f:
                 self.alertSchema.store_alerts(f, alerts)
 
     def _patchDiaSources(self, diaSources):