Coerce label values to strings in MetricFamily add_metric

[cpruijsen]

@csmarchbanks

Fixes #270.

Custom collectors could store integer label values in add_metric(), then fail during exposition with AttributeError: 'int' object has no attribute 'replace'.
This converts the labels sequence to strings in all eight metric families, consistent with Metric.labels().

Chosen: convert at add_metric() entry. The alternative was conversion during exposition.
Entry conversion gives every sample consumer string labels and leaves the escaping code unchanged.

The regression test checks string labels across all eight families and successful Prometheus exposition.
It fails on the baseline with AssertionError: 2 != None.

Validation:

• Python 3.14 full suite: 427 passed, 3 optional dependency skips.
• tests/test_core.py: 115 passed under both Python 3.9 and 3.14.
• Flake8, isort, and the pinned mypy check pass.
• The local Python 3.9 full suite has six TLS 1.3 failures that also occur on the baseline.

[csmarchbanks]

Thanks!

I think we need updates for InfoMetricFamily and StateSetMetricFamily as well which can pass in ints and such via dicts.

Should we also update the type hints to be Sequence[object] as we just need things to be stringable now as well?

[cpruijsen]

Info now converts the values in its dict to str, and StateSet converts the state names. I also widened the hints where values get stringified: the add_metric labels are Sequence[object], the info dict is Mapping[str, object] and the stateset dict is Mapping[Any, bool]. I used Mapping because Dict is invariant, so with Dict mypy would reject existing callers that pass Dict[str, str].