Use the prometheus.type metric metadata to support double-writing unknown-typed metrics

[dashpole]

Depends on open-telemetry/opentelemetry-collector-contrib#32605

This removes the gcp.untypedDoubleExport feature gate, as it is no longer needed.

Specification

From https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/compatibility/prometheus_and_openmetrics.md#gauges-1

An OpenTelemetry Gauge MUST be converted to a Prometheus Unknown-typed metric if the prometheus.type key of metric.metadata is unknown. Otherwise, it MUST be converted to a Prometheus Gauge.

[dashpole]

cc @ridwanmsharif

[dashpole]

Blocked by open-telemetry/opentelemetry-collector#10026

[dashpole]

I pinned the pdata dependency only in the integration test module (so it doesn't affect the released modules) to include open-telemetry/opentelemetry-collector#10026. This makes sure the fixture-based tests continue to work correctly.

[dashpole]

This is ready for review now

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.37%. Comparing base (4caace7) to head (99ac427).
Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #839      +/-   ##
==========================================
+ Coverage   61.03%   62.37%   +1.33%     
==========================================
  Files          56       57       +1     
  Lines        5903     4901    -1002     
==========================================
- Hits         3603     3057     -546     
+ Misses       2143     1687     -456     
  Partials      157      157              

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[ridwanmsharif]

Woooo! So happy we can unfork now