UTF-8 support in metric and label names

[fedetorres93]

Adds UTF-8 support for metric and label names.

These changes are based on the work done on the Prometheus common libraries here and here

• The prometheus-metrics-exposition-formats module will use the new quoting syntax {"foo"} iff the metric does not conform to the legacy name format (foo{})
• The prometheus-metrics-model module has a new flag (NameValidationScheme) that determines if validation is done using the legacy or the UTF-8 scheme. This flag can be set via a property in the properties file.
• Scrapers can announce via content negotiation that they support UTF-8 names by adding escaping=allow-utf-8 in the Accept header. In cases where UTF-8 is not available, metric providers can be configured to escape names in a few different ways: values (U__ UTF value escaping for perfect round-tripping), underscores (all invalid chars become _), dots (dots become _dot_, _ becomes __, all other values become ___). Escaping can either be a global default (PrometheusNaming.nameEscapingScheme) or can also be specified in Accept header with the escaping= term, which can be allow-utf-8 (for UTF-8-compatible), underscores, dots, or values.
  This should still be a noop for existing configurations because scrapers will not be passing the escaping key in the Accept header. Existing functionality is maintained.
• The prometheus-metrics-exporter-pushgateway module will escape UTF-8 grouping keys in the URL path used when pushing metrics (see Add UTF-8 support in metric and label names pushgateway#689)

Work towards prometheus/prometheus#13095

[fedetorres93]

Given the ongoing discussion about unit suffixes for OM 2.0 (prometheus/OpenMetrics#286), I think we can take this UTF-8 work as a basis and then add the necessary changes to comply with the final consensus on suffixes.

[zeitlinger]

Given the ongoing discussion about unit suffixes for OM 2.0 (prometheus/OpenMetrics#286), I think we can take this UTF-8 work as a basis and then add the necessary changes to comply with the final consensus on suffixes.

@fstab are you ok with that?

[fstab]

Update: The client_java maintainers just decided that we'll wait 6 more weeks, until 1 April 2025. If we have OpenMetrics 2.0 by then we will implement that. If OpenMetrics 2.0 is still under discussion in 6 weeks we will merge this PR.

[fedetorres93]

@fstab Good to know, thanks for the update!

[fedetorres93]

Hello @fstab, I just wanted to follow-up on your last comment. Seems like OM 2.0 is still under discussion, so do you think now is a good time to reconsider merging this PR?

[fstab]

We have our client_java community call tomorrow, and can discuss this there. If you have time, feel free to join. See the public Prometheus calendar linked here: https://prometheus.io/community/

[zeitlinger]

@fedetorres93 thanks for the PR!

let me start with some high level questions before an in-depth review:

• I think we need a setting for https://prometheus.io/docs/guides/utf8/#otlp-metrics in ExporterOpenTelemetryProperties
• the statics in PrometheusNaming should be final - e.g. the scrape handler should pass the escaping as an argument