Add @metadata_entrypoint decorator

[florimondmanca]

What does this PR do?

Provide a decorator for automatically performing certain tasks associated to metadata collection.

Motivation

• Provide some standardization to how metadata collection is handled (i.e. encourage a submit-in-a-single-method style).
• Make any automatic processing explicit (via the decorator marker).
• Provide sufficient flexibility to implementers (eg classes can have multiple entrypoints; we can also consider providing a context manager for in-.check() usage without having to create separate methods).

Additional Notes

More explicit alternative to #6078
Refs #5748

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 #6084 into master will increase coverage by 0.41%.
The diff coverage is 97.05%.

Impacted Files	Coverage Δ
datadog_checks_base/tests/test_metadata.py	92.46% <96.00%> (+0.28%)	⬆️
...dog_checks_base/datadog_checks/base/checks/base.py	85.29% <100.00%> (+0.36%)	⬆️
couch/tests/test_couchv2.py	97.34% <0.00%> (ø)
riak/datadog_checks/riak/__about__.py
riak/datadog_checks/riak/__init__.py
kube_dns/datadog_checks/kube_dns/kube_dns.py
kube_dns/datadog_checks/kube_dns/__init__.py
riak/tests/conftest.py
presto/tests/conftest.py
ecs_fargate/datadog_checks/ecs_fargate/__init__.py
... and 225 more

[mgarabed]

In general, not sure if I like this proposal - the extra import and decorator seems pretty heavy for a relatively light functionality benefit.

[florimondmanca]

Updated the PR to reflect the latest state of discussions.

Personally I'm a bit less convinced this is worth it:

• Doing automatic catch-all exception handling actually doesn't sound right (if metadata collection is broken in an integration we should fix it instead of silence exceptions as debug logs).
• This means this would just be an API for an if statement, but it's not easy to backport since not all integrations use the one-method-to-collect-metadata style. (So we might as well just to a pass to verify that .is_metadata_collection_enabled() is checked wherever it makes sense…) At least we'd have it for newer integrations though.

[ofek]

💯

[ofek]

./datadog_checks/base/checks/win/winpdh.py:133:81: W291 trailing whitespace

[florimondmanca]

CI failures are unrelated (E2E flakes, Envoy "No Go files…"), merging…