Limit the number of reported contexts

[hithwen]

The Kafka consumer checks counts the total number of submitted context and displays a warning when the check submits too many metrics.
But the check still collects all metrics, this is a regression introduced here: c4a29b1

On the legacy implementation the limit is applied at collection time. However, as the new implementation uses futures to perform the collection this strategy can only be partially applied and so the limit is applied on submission time.

[codecov]

Codecov Report

Merging #7084 into master will decrease coverage by 7.35%.
The diff coverage is 78.84%.

Impacted Files	Coverage Δ
...er/datadog_checks/kafka_consumer/kafka_consumer.py	71.64% <73.68%> (ø)
...mer/datadog_checks/kafka_consumer/legacy_0_10_2.py	70.38% <73.91%> (ø)
kafka_consumer/tests/test_kafka_consumer.py	96.49% <100.00%> (ø)
disk/tests/test_filter.py
mapreduce/datadog_checks/mapreduce/__init__.py
tokumx/tests/common.py
fluentd/tests/test_unit.py
...g_checks_base/datadog_checks/base/stubs/similar.py
...atadog_checks/kube_controller_manager/__about__.py
...hecks_dev/tests/tooling/configuration/test_load.py
... and 821 more

[ChristineTChen]

Looks good! Left a few comments to make the new log messages consistent.

[FlorianVeaux]

Looks great