opentelemetry-exporter-prometheus: fix wrong labels when metrics share a name

[jbyers44]

Description

Builds each metric family with the union of the label keys of all its data points.

• prometheus_client sets the label names of a family when it creates the family. add_metric then matches values to those names by position. collect() now creates each family only after it has all of that family's points.
• A missing label key gets "", as it already does for points within one metric. Each metric name keeps one HELP and one TYPE line, as in Fix duplicate HELP/TYPE declarations in Prometheus exporter #4869.
• collect() now yields target_info directly, because the dict now holds _FamilyBuilder objects, not finished families.
• If more than one batch is queued when a scrape starts, the output has duplicate HELP, TYPE, and series lines, as on main.

Fixes #5685

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Added test_metrics_with_same_name_and_different_label_keys, which checks the full output in both record orders. It fails on main and passes with the fix.
• test_multiple_data_points_with_different_label_sets still passes.
• py312-test-opentelemetry-exporter-prometheus (38 tests), lint-opentelemetry-exporter-prometheus, and precommit pass.
• The reproduction script in Prometheus exporter puts label values under the wrong label names when two instruments share a name #5685 prints the Expected Result in both record orders.

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

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

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Pull request dashboard status

Waiting on the author · refreshed 2026-09-25 17:13 UTC

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