Check `float` type hints in Prometheus backend

`count()` and `distribution_cut()` in `slo_generator/backends/prometheus.py` return `float` and `Tuple[float, float]`, respectively, while counts are expected to be `int`.

Check exactly what is returned by the Prometheus API, for example by setting breakpoints and inspecting values when querying a local containerized Prometheus, and confirm whether (and why) we need to convert the response to `float`.