Prepare release v0.126.0

[github-actions]

Description

• Release Helm chart version 0.126.0
• Includes collector version 0.126.0

[jvoravong]

--- FAIL: Test_ControlPlaneMetrics (60.70s)
--- FAIL: Test_ControlPlaneMetrics/kubernetes-scheduler_histograms=true (30.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
panic: runtime error: invalid memory address or nil pointer dereference

--- FAIL: Test_IstioMetrics (112.14s)
--- FAIL: Test_IstioMetrics/istiod_metrics_captured (33.05s)
--- FAIL: Test_IstioMetrics/istio_ingress_metrics_captured (31.14s)

[jvoravong]

Validated that Istio tests pass when agent.featureGates=-receiver.prometheusreceiver.RemoveLegacyResourceAttributes is set. The test failures are due to the promotion of this feature gate to beta (enabled by default).

To address this, we need to:

• Regenerate the expected Istio test results with the feature gate enabled to account for breaking changes.
  • Running the tests with the env var UPDATE_EXPECTED_RESULTS can be used in github workflows or locally to generate new test results.
• Add a breaking change entry in CHANGELOG.md and document the changes in UPGRADING.md, including instructions for configuring and using the previous functionality.
• Ensure CHANGELOG.md links to UPGRADING.md.

References:

• Splunk Collector CHANGELOG.md
• OpenTelemetry Collector Contrib CHANGELOG.md

[jvoravong]

When updating expected test results, try to limit changes to Istio-related files only. Avoid modifying unrelated files unless necessary—this helps keep all tests passing and preserves clean git history for easier blame tracking.

[dmitryax]

LGTM