Add flag to enable high-cardinality HTTP duration labels

[aliaqel-stripe]

Provide a description of what has been changed

This adds --enable-high-cardinality-metrics-labels to the KEDA operator, defaulting to false.

When the flag is enabled, keda_scaler_http_request_duration_seconds includes the high-cardinality namespace, scaled_resource, trigger_name, and metric_name labels in addition to the existing low-cardinality labels. When the flag is disabled, the histogram is still emitted with only scaler and status_code labels.

The existing scaler HTTP request counter labels are unchanged, and the internal Metrics Service gRPC histograms are no longer gated by this flag.

This also updates unit and e2e coverage for both label modes and updates the changelog.

The e2e coverage was also hardened so the new flag assertions are reliable:

• deployment arg mutations now wait for a full rollout, so the tests do not assert against old operator pods after toggling the flag
• the OpenTelemetry disabled-path assertion now runs before the Prometheus HTTP-client scenarios, so the collector validates the low-cardinality histogram shape before the high-cardinality path runs
• the OpenTelemetry scaler error assertion now waits for the collector export instead of relying on a fixed sleep

Checklist

• I have verified that my change is according to the deprecations & breaking changes policy
• Tests have been added (if applicable)
• Changelog has been updated and is aligned with our changelog requirements, only when the change impacts end users
• A PR is opened to update our Helm chart (repo) (if applicable, ie. when deployment manifests are modified)
• A PR is opened to update the documentation on (repo) (if applicable)
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

Fixes #7731

Relates to kedacore/keda-docs#1772, kedacore/charts#855

[github-actions]

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[JorTurFer]

/ar test code