Document high-cardinality HTTP duration label flag

[aliaqel-stripe]

Provide a description of what has been changed

This documents the new --enable-high-cardinality-metrics-labels flag for the KEDA operator.

The flag defaults to false. When enabled, scaler HTTP request duration metrics include the high-cardinality namespace, scaled_resource, trigger_name, and metric_name labels/attributes. When disabled, the duration histogram is still emitted with low-cardinality labels such as scaler and status_code.

The scaler HTTP request counters are unchanged, and internal Metrics Service gRPC histograms are not controlled by this flag.

Relates to kedacore/keda#7731
Relates to kedacore/keda#7751

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Fixes #

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