[Observability] Emit metric ClustermgtdHeartbeat to signal clustermgtd heartbeat.

[gmarciani]

Description of changes

Emit metric ClustermgtdHeartbeat to signal clustermgtd heartbeat.
The metric is emitted with dimensions: ClusterName and InstanceId.

The metric is intentionally emitted at the end of the clustermgtd loop to represent the real health of the daemon.
If it was emitted at the beginning of the iteration, it would be open to false negatives.

This PR depends on the permissions added in aws/aws-parallelcluster#7209

Tests

• Unit tests (extended to cover the current changes)
• Manually verified by creating a cluster and checking that (i) the metric is pushed (ii) when the metric push fails (manually removing the permissions to push it), the overall clustermgtd iteration is not compromised.

References

1. The max length for a metric name is 255 chars. See https://docs.aws.amazon.com/AmazonCloudWatch/latest/APIReference/API_MetricDatum.html

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[himani2411]

LGTM!