docs: Add examples to docstrings

[msto]

Add usage examples to docstrings for _typing_extensions helper functions, CounterPivotTable, DelimitedList, Metric.read(), Metric._header_fieldnames(), and MetricWriter.

[clintval]

If you add pytest-doctestplus to the dev dependencies.

Then have something like:

[tool.pytest.ini_options]
minversion = "7.4"
addopts    = [
    "--color=yes",
    "--cov-fail-under=80",
    "--cov-report=term-missing",
    "--cov",
    "--cov=typeline",
    "--doctest-modules",
    "--doctest-plus",
    "--import-mode=importlib",
]
doctest_plus = "enabled"
doctest_optionflags = [
    "ELLIPSIS",
    "IGNORE_EXCEPTION_DETAIL",
    "NORMALIZE_WHITESPACE",
]

You can get auto-testing of these examples.

If you have any Python in Markdown, you can test too with:

[tool.poe.tasks]
check-tests = "pytest --doctest-glob='*.md'"