Do not fail on octet stream content type for OpenMetrics

[AlexandreYang]

What does this PR do?

1. Set encoding to utf-8 when there is no encoding to avoid failure down the road.

Content might be in bytes when the service exposing the openmetrics endpoint doesn't set correctly the Content-Type and Encoding.

The openmetrics/prometheus exposition format suggest using utf-8: https://github.com/prometheus/docs/blob/master/content/docs/instrumenting/exposition_formats.md

2. Explicitly set the content type we accept to text/plain

Motivation

Currently, if bytes content is received. In Python 3 (works for Python 2), the openmetrics check will fail with the following issue:

openmetrics (1.3.0)
-------------------
  Instance ID: openmetrics:idm-mhcf74:efee10b119b1d5b0 [ERROR]
  Configuration Source: file:/etc/datadog-agent/conf.d/openmetrics.yaml
  Total Runs: 13
  Metric Samples: Last Run: 0, Total: 0
  Events: Last Run: 0, Total: 0
  Service Checks: Last Run: 1, Total: 13
  Average Execution Time : 11ms
  Error: startswith first arg must be bytes or a tuple of bytes, not str
  Traceback (most recent call last):
    File "/opt/datadog-agent/embedded/lib/python3.7/site-packages/datadog_checks/base/checks/base.py", line 678, in run
      self.check(instance)
    File "/opt/datadog-agent/embedded/lib/python3.7/site-packages/datadog_checks/base/checks/openmetrics/base_check.py", line 82, in check
      self.process(scraper_config)
    File "/opt/datadog-agent/embedded/lib/python3.7/site-packages/datadog_checks/base/checks/openmetrics/mixins.py", line 378, 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 352, in scrape_metrics
      for metric in self.parse_metric_family(response, scraper_config):
    File "/opt/datadog-agent/embedded/lib/python3.7/site-packages/datadog_checks/base/checks/openmetrics/mixins.py", line 301, in parse_metric_family
      for metric in text_fd_to_metric_families(input_gen):
    File "/opt/datadog-agent/embedded/lib/python3.7/site-packages/prometheus_client/parser.py", line 138, in text_fd_to_metric_families
      if line.startswith('#'):
  TypeError: startswith first arg must be bytes or a tuple of bytes, not str

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 #5843 into master will increase coverage by 0.09%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
disk/tests/common.py	63.63% <0.00%> (-36.37%)	⬇️
disk/tests/conftest.py	85.71% <0.00%> (-11.43%)	⬇️
disk/tests/test_filter.py	96.45% <0.00%> (-3.55%)	⬇️
disk/datadog_checks/disk/disk.py	83.88% <0.00%> (-2.07%)	⬇️
airflow/tests/conftest.py	100.00% <0.00%> (ø)	⬆️
ibm_mq/datadog_checks/ibm_mq/ibm_mq.py
nagios/datadog_checks/nagios/nagios.py
kube_apiserver_metrics/tests/conftest.py
exchange_server/tests/common.py
activemq_xml/tests/conftest.py
... and 364 more

[AlexandreYang]

Closing in favour of #5847

[xornivore]

👍