fix(config): fix regression in config merge

[TheMeier]

Fixes: #5456

Pull Request Checklist

Please check all the applicable boxes.

• Please list all open issue(s) discussed with maintainers related to this change
  • Fixes regression in merging global notifier configs #5456
• Is this a new Receiver integration?
• Is this a bugfix?
  • I have added tests that can reproduce the bug which pass with this bugfix applied
• I have signed-off my commits
• I will follow best practices for contributing to this project

Which user-facing changes does this PR introduce?

[BUGFIX] Config: correct merging of global configs for opsgenie, victorops and rocketchat notifiers

[coderabbitai]

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 5ed6877 and a9c0eac.

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• config/config.go

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📝 Walkthrough

Walkthrough

Config.UnmarshalYAML now merges global settings into OpsGenie, VictorOps, and Rocket.Chat receivers without replacing local credentials. Tests cover inline and file credential precedence.

Changes

Notifier configuration merge

Layer / File(s)	Summary
Receiver defaulting and credential precedence config/config.go	OpsGenie, VictorOps, and Rocket.Chat receivers inherit global settings only when receiver-level values are absent. Existing validation and URL normalization remain in place.
Credential precedence validation config/config_test.go	Tests verify that local inline credentials override global file credentials, and local file credentials override global inline credentials. Rocket.Chat tests cover token and token ID fields.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to a9c0e

This change corrects configuration merging for several notifier integrations, with no actionable merge-blocking risk remaining beyond normal checks and review.

Possibly related PRs

• prometheus/alertmanager#5320: Both PRs modify OpsGenie receiver configuration handling in config/config.go.
• prometheus/alertmanager#5384: Both PRs modify Rocket.Chat receiver inheritance and credential handling.
• prometheus/alertmanager#5395: Both PRs modify defaulting and validation behavior for the same notifier configurations.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies a configuration merge regression fix, which matches the primary changes.
Description check	✅ Passed	The description identifies issue #5456, marks the bugfix tests, records sign-off, and includes appropriate release notes.
Linked Issues check	✅ Passed	The changes correct global configuration merging for OpsGenie, VictorOps, and Rocket.Chat and add regression tests as required by [#5456].
Out of Scope Changes check	✅ Passed	The changes are limited to notifier configuration merging and related regression tests, with no unrelated scope detected.

✨ Finishing Touches

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• Create PR with unit tests

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[SoloJacobs]

Can we restrict this change to the lines, which are actually problematic? :-)

[SoloJacobs]

This seems like a noop? I like cmp.Or.

[TheMeier]

yes, the revert is really not necessary for c.Global.HTTPConfig. Changed