Added validation functions for opmon metrics

[bieryAtFnal]

Description

These changes are for fddaq-v5.6.0...

As part of adding the validation of opmon metrics to integtests (DUNE-DAQ/daq-deliverables#202), I created several utility functions in a new file named opmon_metric_checks.py. There is a function to collate a set of metric files on disk into a single JSON structure and a couple of validation functions - one to check whether the number of metric samples is within a specified range and one to check whether the sum of the metric values is within a specified range.

These changes also include a bug fix - the directory glob to find the metric files produced in an integtest needed to be adjusted.

The way to test these changes is to run the modified regression tests in the daqsystemtest repo (which are part of a correlated PR).

Type of change

• Documentation (non-breaking change that adds or improves the documentation)
• New feature or enhancement (non-breaking change which adds functionality)
• Optimization (non-breaking change that improves code/performance)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (whatever its nature)

Testing checklist

• Unit tests pass (e.g. dbt-build --unittest)
• Minimal system quicktest passes (pytest -s minimal_system_quick_test.py)
• Full set of integration tests pass (daqsystemtest_integtest_bundle.sh)
• Python tests pass if applicable (e.g. python -m pytest)
• Pre-commit hooks run successfully if applicable (e.g. pre-commit run --all-files)

Further checks

• Code is commented where needed, particularly in hard-to-understand areas
• Code style is correct (dbt-build --lint, and/or see https://dune-daq-sw.readthedocs.io/en/latest/packages/styleguide/)
• If applicable, new tests have been added or an issue has been opened to tackle that in the future.