Update prometheus metric exporter to comply with data model specification

[srikanthccv]

The exporter should perform this such as

1. The Name of an OTLP metric MUST be added as the OpenMetrics MetricFamily Name, with unit and type suffixes added as described below. The metric name is required to match the regex: a-zA-Z_:*. Invalid characters in the metric name MUST be replaced with the _ character. Multiple consecutive _ characters MUST be replaced with a single _ character.
   • Additionally, the unit MUST be added as a suffix to the metric name, and SHOULD be converted to base units recommended by OpenMetrics when possible. The unit suffix comes before any type-specific suffixes.
2. The data point type of an OTLP metric MUST be added as OpenMetrics TYPE metadata.

Link to spec: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/data-model.md#otlp-metric-points-to-prometheus

[dashpole]

New spec link: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/compatibility/prometheus_and_openmetrics.md#otlp-metric-points-to-prometheus

Looking through the current implementation, it looks like the name is already sanitized. Unit suffixes are already added by the upstream library. TYPE metadata is already added by upstream as well.

Remaining items (from the comment above):

• repeated _ are not replaced with a single _.
• I would hold off on converting to base units (e.g. milliseconds to seconds) for now, but we should convert from abbreviations to full words when possible (e.g. ms to milliseconds).

[nstawski]

I would like to take this issue.

[XuanWang-Amos]

Hi, is there any update on the progress? We're running into some issue while exporting metrics to prometheus because they don't allow {} in unit, based on current specification, unit within brackets should be dropped.

[aabmass]

@dashpole since this is a breaking change to metric names, I'd like to minimize the disruption by fixing as much as possible at once. I see the spec also has

• Converting "foo/bar" to "foo_per_bar".

But I don't see it in the Go impl. Should I be implementing this case as well?

[dashpole]

You can skip it if it isn't in other SDK implementations.

[dashpole]

But here is the collector's implementation of foo_per_bar: https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main/pkg/translator/prometheus/normalize_name.go

[aabmass]

I implemented the "per" conversion in this PR as well