[Logs] Add log lines to clustermgtd to signal that heartbeat file has been update and heartbeat metric has been published.

[gmarciani]

Description of changes

Add log lines to clustermgtd to signal that heartbeat file has been update and heartbeat metric has been published.

Tests

Verified logs are written and wre actually helpful in root causing a problem.

2026-02-03 23:13:42,879 - [slurm_plugin.clustermgtd:_terminate_orphaned_instances] - INFO - Checking for orphaned instance
2026-02-03 23:13:42,879 - [slurm_plugin.clustermgtd:manage_cluster] - INFO - Writing heartbeat file
2026-02-03 23:13:42,880 - [slurm_plugin.clustermgtd:manage_cluster] - INFO - Publishing heartbeat metric
2026-02-03 23:29:43,747 - [slurm_plugin.cloudwatch_utils:put_metric] - ERROR - Failed to publish metric ClustermgtdHeartbeat: Connect timeout on endpoint URL: "https://monitoring.us-east-1.amazonaws.com/"
2026-02-03 23:29:43,747 - [slurm_plugin.clustermgtd:_get_config] - INFO - Reading /etc/parallelcluster/slurm_plugin/parallelcluster_clustermgtd.conf
2026-02-03 23:29:43,750 - [slurm_plugin.clustermgtd:manage_cluster] - INFO - Managing cluster...

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