Fix the documentation of the metrics setting

[L3n41c]

What does this PR do?

Fix the documentation of the metrics parameter of the Openmetrics v2 check.
It’s a regular expression and not a glob pattern as it used to be with the the Prometheus v1 check.

Motivation

With the Prometheus v1 check, metrics seemed to be a glob pattern and this configuration was working:

=== openmetrics check ===
Configuration provider: kubernetes
Configuration source: kubelet:containerd://543f4a8a2170333ca3efcd9fbb20c41f9ace1ea9e793925fb44a60ce71579265
Instance ID: openmetrics:openmetrics_gen:8329dd0b4377b5be
metrics:
- '*'
namespace: openmetrics_gen
prometheus_url: http://10.248.7.124:8080/metrics

But this Openmetrics v2 configuration produces the following error while trying to compile the regular expression:

=== openmetrics check ===
Configuration provider: kubernetes
Configuration source: kubelet:containerd://edebeab89d5d1bffdfdabb3cb43b64cc97d08476ca2a5d6f46d32058f30a130f
Instance ID: openmetrics:openmetrics_gen:cd9516f0abeeacdc
metrics:
- '*'
namespace: openmetrics_gen
openmetrics_endpoint: http://10.248.8.9:8080/metrics

      Error: nothing to repeat at position 0
      Traceback (most recent call last):
        File "/opt/datadog-agent/embedded/lib/python3.8/site-packages/datadog_checks/base/checks/base.py", line 992, in run
          initialization()
        File "/opt/datadog-agent/embedded/lib/python3.8/site-packages/datadog_checks/base/checks/openmetrics/v2/base.py", line 70, in configure_scrapers
          scrapers[endpoint] = self.create_scraper(config)
        File "/opt/datadog-agent/embedded/lib/python3.8/site-packages/datadog_checks/base/checks/openmetrics/v2/base.py", line 77, in create_scraper
          return OpenMetricsScraper(self, self.get_config_with_defaults(config))
        File "/opt/datadog-agent/embedded/lib/python3.8/site-packages/datadog_checks/base/checks/openmetrics/v2/scraper.py", line 63, in __init__
          self.metric_transformer = MetricTransformer(self.check, config)
        File "/opt/datadog-agent/embedded/lib/python3.8/site-packages/datadog_checks/base/checks/openmetrics/v2/transform.py", line 50, in __init__
          self.metric_patterns.append((re.compile(raw_metric_name), config))
        File "/opt/datadog-agent/embedded/lib/python3.8/re.py", line 252, in compile
          return _compile(pattern, flags)
        File "/opt/datadog-agent/embedded/lib/python3.8/re.py", line 304, in _compile
          p = sre_compile.compile(pattern, flags)
        File "/opt/datadog-agent/embedded/lib/python3.8/sre_compile.py", line 764, in compile
          p = sre_parse.parse(p, flags)
        File "/opt/datadog-agent/embedded/lib/python3.8/sre_parse.py", line 948, in parse
          p = _parse_sub(source, state, flags & SRE_FLAG_VERBOSE, 0)
        File "/opt/datadog-agent/embedded/lib/python3.8/sre_parse.py", line 443, in _parse_sub
          itemsappend(_parse(source, state, verbose, nested + 1,
        File "/opt/datadog-agent/embedded/lib/python3.8/sre_parse.py", line 668, in _parse
          raise source.error("nothing to repeat",
      re.error: nothing to repeat at position 0

Replacing * by .* like this fixes the issue:

=== openmetrics check ===
Configuration provider: kubernetes
Configuration source: kubelet:containerd://70fa32bc8ce659f45c5f2cdf59b2a398ac21ad42de60e9ed9777691f102de85b
Instance ID: openmetrics:openmetrics_gen:27e2f38a7481e8a1
metrics:
- .*
namespace: openmetrics_gen
openmetrics_endpoint: http://10.248.7.123:8080/metrics

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

[urseberry]

I wonder if saying that the field allows regular expressions will cause customers to input arbitrary regular expressions?