Adds a test to verify that the metrics returned with the prometheus text
format are formatted as expected.

The prometheus text protocol requires that the label value to be
escaped. Note that this is only needed for the text protocol and not for
the protobuf-based prometheus protocol.

Without this sanitization, a single label with one of these special
characters makes the whole metrics output unparseable by prometheus.

> label_value can be any sequence of UTF-8 characters, but the backslash
> (\), double-quote ("), and line feed (\n) characters have to be
> escaped as \\, \", and \n, respectively.

From https://github.com/prometheus/docs/blob/main/content/docs/instrumenting/exposition_formats.md

This avoids a string copy when writing the metric value and the metric
type to the prometheus text-based response.

Note that now `fmt::print` is used to format the values instead of the
originally used `std::to_string`. This is because `std::to_string` is
going to change its formatting behaviour in C++26, so we prefer to use a
more deterministic formatter here.

The formatting of the numbers matters because if the floating point
numbers are converted to scientific notation, they can lose precision.

Ref: https://en.cppreference.com/w/cpp/string/basic_string/to_string