Improve assertion messages of aggregator stub

[AlexandreYang]

What does this PR do?

Improve assertion messages

Motivation

Before

        if count is not None:
            assert len(candidates) == count
        else:
>           assert len(candidates) >= at_least
E           assert 0 >= 1
E            +  where 0 = len([])

After

E           AssertionError: Candidates size assertion for clickhouse.cache_dictionary.update_queue.batches, count: None, at_least: 1) failed
E           assert 0 >= 1
E            +  where 0 = len([])

Additional Notes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

[codecov]

Codecov Report

Merging #5975 into master will decrease coverage by 26.98%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
...ts/tooling/configuration/consumers/test_example.py
...checks_dev/tests/plugin/test_environment_runner.py
oracle/tests/test_oracle.py
...r/datadog_checks/openstack_controller/__about__.py
twistlock/datadog_checks/twistlock/twistlock.py
linkerd/datadog_checks/linkerd/linkerd.py
vsphere/datadog_checks/vsphere/cache.py
btrfs/tests/test_btrfs.py
...nmetrics/datadog_checks/openmetrics/openmetrics.py
linkerd/datadog_checks/linkerd/metrics.py
... and 757 more

[codecov]

Codecov Report

Merging #5975 into master will increase coverage by 0.23%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
couch/tests/conftest.py	85.54% <0.00%> (-1.21%)	⬇️
couch/datadog_checks/couch/couch.py	88.21% <0.00%> (-1.15%)	⬇️
...er/datadog_checks/openstack_controller/settings.py
statsd/datadog_checks/statsd/__about__.py
pgbouncer/datadog_checks/pgbouncer/__about__.py
...r/datadog_checks/openstack_controller/__about__.py
openstack_controller/tests/common.py
openstack_controller/tests/test_openstack.py
twistlock/datadog_checks/twistlock/twistlock.py
pgbouncer/tests/common.py
... and 309 more

[prognant]

👍