feat(exporter-prometheus): add config option for default aggregation

[ArthurSens]

Which problem is this PR solving?

Adds support for configuring the Prometheus exporter's MetricReader default aggregation as a function of instrument kind.
This implements the behavior described in the Prometheus exporter spec section being stabilized in open-telemetry/opentelemetry-specification#5113

Fixes # (issue)

Short description of the changes

• Adds a defaultAggregation option to PrometheusExporter config.
• Passes the configured aggregation selector through to the underlying MetricReader.
• Keeps the existing default behavior by using SDK default aggregation when no option is configured.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• npm run compile --workspace=@opentelemetry/exporter-prometheus
• npm run lint --workspace=@opentelemetry/exporter-prometheus -- --quiet
• npm test -- --grep "defaultAggregation|default aggregation|configured defaultAggregation"

Checklist:

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

---

Disclaimer: This PR was assisted by LLMs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.95%. Comparing base (00aed14) to head (5d416ac).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6761   +/-   ##
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  Coverage   94.95%   94.95%           
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  Files         377      377           
  Lines       12743    12744    +1     
  Branches     2896     2897    +1     
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+ Hits        12100    12101    +1     
  Misses        643      643           

Files with missing lines	Coverage Δ
...etry-exporter-prometheus/src/PrometheusExporter.ts	93.75% <100.00%> (+0.06%)	⬆️

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[pichlermarc]

Sorry for the late review on this PR. I think having AggreagationSelector be a function was a mistake we made, since it implies that changing the returned values after configuring the MetricReader (in this case PrometheusExporter) would re-configure already-created instruments, which is not the case.

Suggestion: let's implement this feature in a why that there's an aggregationPreference option that people can configure like so:

new PrometheusExporter({
  aggregationPreference: {
     gauge: {
       // this here is an AggregationOption
       type: AggregationType.LAST_VALUE
     },
     // non-specified ones fall back to default
  }
})

Internally, the aggregationPreference is converted to an aggregationSelector and published this way. This way it's clearer that nothing can be changed once the exporter has been instantiated. :)