Thanks @fedetorres93!

There is already support for dots in metric and label names in client_java. It will be easy to extend this to other characters. The motivation for allowing dots was to support metric/label names defined in the OpenTelemetry semantic conventions.

Currently dots are only exposed in OpenTelemetry format. In Prometheus text format, OpenMetrics text format, and OpenMetrics protobuf format dots are replaced with underscores.

I assume for UTF-8 characters in Prometheus format we will define a new OpenMetrics version, right?

I think the following two considerations make sense:

When converting OpenTelemetry names to Prometheus names follow the rules defined here: https://opentelemetry.io/docs/specs/otel/compatibility/prometheus_and_openmetrics/. These are the rules that are also implemented in the OpenTelemetry collector. For a user it should not matter whether they scrape Prometheus format, or whether they push OpenTelemetry format and have a collector convert to Prometheus remote write. The resulting metric and attribute names should be the same, therefore Prometheus client libraries should implement the OpenTelemetry standard for converting arbitrary names to Prometheus names.
Prometheus client libraries have a "fail fast" approach: When you register metrics with conflicting names, registration fails. We don't defer these errors to scrape time. I think we should look at the classic Prometheus names when checking for conflicts, i.e. we should fail if a user registers a metric named requests.total and then registers a metric named requests_total. While this might theoretically work when exposing new names only, it will fail at scrape time for older Prometheus servers. We should consider this bad practice and prevent this in our client libraries.

What do you think? If you feel we should have a small "client library support for UTF-8" proposal with the points above I'm happy to write one.

Implement UTF8 Support #916

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