[datadog_cluster_agent] Add local failover workload store metric

[zhuminyi]

What does this PR do?

Two metrics are added
'datadog.cluster_agent.autoscaling_workload_store_load_entities' for tracking the total number of entities in autoscaling workload store
'datadog.cluster_agent.autoscaling_workload_store_job_queue_length' for tracking the pending job queue len

Motivation

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.27%. Comparing base (c42d9ec) to head (bd822cd).
Report is 9 commits behind head on master.

Additional details and impacted files

Flag	Coverage Δ
activemq	?
cassandra	?
datadog_cluster_agent	90.19% <ø> (ø)
hive	?
hivemq	?
hudi	?
ignite	?
jboss_wildfly	?
kafka	?
presto	?
solr	?

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