feat(opentelemetry-exporter-prometheus): add translation strategy support

[cjihrig]

Which problem is this PR solving?

The Prometheus exporter does not currently support Translation Strategy.

Related to #6605

Short description of the changes

This PR adds support for Translation Strategy in a way that can also work with Content Negotiation in the future if necessary.

I am opening as a draft because I need to add more tests for the non-default behavior. There is also the issue of adding unit suffixes. I have another branch that implements unit suffixes, but turning them on by default (which is the spec behavior) would be a breaking change, so that will need to be figured out too.

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Existing and new tests

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added
• Documentation has been updated

[codecov]

Codecov Report

❌ Patch coverage is 89.23077% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.50%. Comparing base (acc9ecd) to head (f8d727a).

Files with missing lines	Patch %	Lines
...ry-exporter-prometheus/src/PrometheusSerializer.ts	90.90%	5 Missing ⚠️
...etry-exporter-prometheus/src/PrometheusExporter.ts	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6653      +/-   ##
==========================================
- Coverage   95.54%   95.50%   -0.04%     
==========================================
  Files         372      373       +1     
  Lines       12339    12377      +38     
  Branches     2829     2850      +21     
==========================================
+ Hits        11789    11821      +32     
- Misses        550      556       +6     

Files with missing lines	Coverage Δ
...ntelemetry-exporter-prometheus/src/export/types.ts	100.00% <100.00%> (ø)
...etry-exporter-prometheus/src/PrometheusExporter.ts	94.94% <77.77%> (-1.76%)	⬇️
...ry-exporter-prometheus/src/PrometheusSerializer.ts	94.35% <90.90%> (-1.43%)	⬇️

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[ArthurSens]

Hello, hello, thanks for starting the effort!

I don't know JS at all, so I can't do a very complete review.

Looking at your tests, and comparing them with the ones we did for go (https://github.com/prometheus/otlptranslator/blob/main/metric_namer_test.go), it seems like a lot of edge cases aren't covered here. Is it worth covering them?

[ArthurSens]

Could you clarify why this needs to be sanitized even if the translation strategy says no escaping?

[cjihrig]

Sure. These constants (ATTR_OTEL_SCOPE_NAME, etc.) have dots in them. According to this part of the spec:

Prometheus exporters MUST by default add the scope name as the otel_scope_name label, the scope version as the otel_scope_version label, the scope schema URL as the otel_scope_schema_url label, the scope attributes as labels with otel_scope_ prefix and following the rules described in the Metric Attributes section below, on all metric points, based on the scope the original data point was nested in.

To me, that is a bit ambiguous, but seems like the names should have underscores. It also seems that Golang unconditionally sanitizes these. See this snapshot for example, where dots are preserved everywhere but the scope attributes.

[cjihrig]

it seems like a lot of edge cases aren't covered here. Is it worth covering them?

I can add more tests. Are there any in particular that you think are worth porting (since that file is 1200+ lines and this PR doesn't implement all of that functionality)? It's also worth noting that I have a separate branch to add unit suffix support which would bring additional tests.

[ArthurSens]

Yep, 1200 LOC is a lot 😅. Let me try to parse that into human-readable text:

Underscore Scaping:

• metric@with#special$chars -> metric_with_special_chars
• 123metric -> _123metric (metric names starting with digit are unnallowed unless UTF-8 is enabled)
• `` (empty) -> Should error
• metric@@##$$name -> metric_name (multiple special characters become a single underscore)
• @#$% -> Should error since it's only special characters, translating into a single underscore

No underscore scaping:

• The cases above are allowed untransformed, besides the empty string

[JacksonWeber]

I think the separator inside the braces might be off here. When the metric name is quoted (UTF-8 form), Prometheus expects a comma between the name and the rest of the labels, not a space:

{"test.dotted_total",otel_scope_name="test"} 1

The current output (and what the new tests assert) is:

{"test.dotted_total" otel_scope_name="test"} 1

which I don't think Prometheus/PromQL parsers will accept. Worth double checking against the exposition format spec and updating both stringify() and the new test fixtures if it's indeed wrong.

[cjihrig]

Good catch. I'll fix this.

[cjihrig]

@ArthurSens thank you for the feedback. I've added those tests, but I did it in a separate PR (#6727) because even without the translation strategy, I believe there may be a few edge case bugs in the existing implementation.