Fix http handler

[AlexandreYang]

What does this PR do?

Before, if the prometheus_url is changed after the scrapper is created, we won't be able to find the handler. Hence leading to a KeyError.

Initial feature PR: #5414

Example:

Average Execution Time : 223ms
      Error: 'https://10.128.246.166:10250/metrics/cadvisor'
      Traceback (most recent call last):
        File "/opt/datadog-agent/embedded/lib/python3.7/site-packages/datadog_checks/base/checks/base.py", line 673, in run
          self.check(instance)
        File "/opt/datadog-agent/embedded/lib/python3.7/site-packages/datadog_checks/kubelet/kubelet.py", line 285, in check
          self.process(self.cadvisor_scraper_config, metric_transformers=self.transformers)
        File "/opt/datadog-agent/embedded/lib/python3.7/site-packages/datadog_checks/base/checks/openmetrics/mixins.py", line 401, in process
          for metric in self.scrape_metrics(scraper_config):
        File "/opt/datadog-agent/embedded/lib/python3.7/site-packages/datadog_checks/base/checks/openmetrics/mixins.py", line 359, in scrape_metrics
          response = self.poll(scraper_config)
        File "/opt/datadog-agent/embedded/lib/python3.7/site-packages/datadog_checks/base/checks/openmetrics/mixins.py", line 559, in poll
          response = self.send_request(endpoint, scraper_config, headers)
        File "/opt/datadog-agent/embedded/lib/python3.7/site-packages/datadog_checks/base/checks/openmetrics/mixins.py", line 583, in send_request
          return self.http_handlers[scraper_config['prometheus_url']].get(endpoint, stream=True, **kwargs)
      KeyError: 'https://10.128.246.166:10250/metrics/cadvisor'

Motivation

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 #5434 into master will increase coverage by <.01%.
The diff coverage is 100%.

Impacted Files	Coverage Δ
...e/datadog_checks/base/checks/openmetrics/mixins.py	75.59% <100%> (-0.23%)	⬇️
...tadog_checks/base/checks/openmetrics/base_check.py	91.83% <100%> (ø)	⬆️
datadog_checks_base/tests/test_openmetrics.py	99.03% <100%> (+0.01%)	⬆️
vsphere/tests/test_vsphere.py	99.02% <0%> (-0.25%)	⬇️
aerospike/tests/test_aerospike.py
postgres/tests/common.py
varnish/datadog_checks/varnish/__about__.py
mesos_master/tests/test_check.py
postgres/datadog_checks/postgres/util.py
... and 124 more

[AlexandreYang]

Note: Let's not expose the http handlers and use get_http_handler

[mfpierre]

LGTM