chore: support dots in label names

[kolesnikovae]

We're currently using prometheus data model for labels (including those that are set as pprof tags) for consistency with other products:

Labels may contain ASCII letters, numbers, as well as underscores. They must match the regex [a-zA-Z_][a-zA-Z0-9_]*.

Although we can not easily loosen the requirement, I'm not very sure if discarding profiles that violate it is the best strategy.

There is ongoing work on support for UTF-8 labels in Prometheus:
prometheus/prometheus#13095:

OTel allows UTF-8 in label names while Prometheus has a much more restrictive set. This is causing friction for users when using Prometheus. In particular, . (dot) is a very common character in OTel and we convert that to _ when converting to Prometheus. For example, service.version becomes service_version.

Until the support is fully implemented, I propose to do the same in Pyroscope – convert . to _ in label names to make interoperability easier. Theoretically, we could replace any disallowed characters, but that leads to ambiguity in certain cases and may confuse users.

[korniltsev]

lgtm

[simonswine]

Thanks for fixing/looking into this. LGTM

A small concern not necessarily blocking, maybe we slightly adapt the output, or that we modified label names with . to _, if we did. Just wonder about your opinion.