Fix decumulating bucket on multiple contexts

[mfpierre]

What does this PR do?

Fix overidding buckets when they have different tag context

Motivation

Support histograms having a cardinality > 1

Additional Notes

Anything else we should know when reviewing?

Review checklist (to be filled by reviewers)

• 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
• Feature or bugfix must have tests
• Git history must be clean
• If PR adds a configuration option, it must be added to the configuration file.

[codecov]

Codecov Report

Merging #4446 into master will decrease coverage by 3.02%.
The diff coverage is 97.72%.

Impacted Files	Coverage Δ
datadog_checks_base/tests/test_openmetrics.py	98.91% <100%> (+0.04%)	⬆️
...e/datadog_checks/base/checks/openmetrics/mixins.py	77.26% <95.83%> (+0.37%)	⬆️
...checks_dev/tests/plugin/test_environment_runner.py
riakcs/datadog_checks/riakcs/riakcs.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
...oller/datadog_checks/openstack_controller/utils.py
istio/tests/test_e2e.py
... and 744 more

[clamoriniere]

just a small comment on the unit-test part

[albertvaka]

If you want to include this in rc3, you can still delete & recreate the tag to include it (rc3 hasn't been built yet).

[ofek]

    def _compute_bucket_hash(self, tags):
        # we need the unique context for all the buckets
        # hence we ignore the `le` tag
        return hash(frozenset(sorted((k, v) for k, v in iteritems(tags) if k != 'le')))