feat: add metric server performance metrics

[kshitizlohia1994]

Add Prometheus and OpenTelemetry performance metrics for the HPA external metrics path on keda-metrics-apiserver and GetMetrics gRPC handling on keda-operator.

Checklist

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• Tests have been added (if applicable)
• Ensure make generate-scalers-schema has been run to update any outdated generated files
• Changelog has been updated and is aligned with our changelog requirements, only when the change impacts end users
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Fixes #3120

Relates to kedacore/keda-docs#1804

Summary

• Metrics adapter: records keda_external_metrics_provider_* Prometheus metrics (and optional OTEL via --enable-opentelemetry-metrics) for each HPA external metric request
• Operator: records keda_metricsservice_get_metrics_* Prometheus/OTEL metrics for each GetMetrics gRPC call
• Adds unit tests in pkg/metricscollector and e2e assertions in prometheus/opentelemetry metrics tests

Test plan

• go test ./pkg/metricscollector/...
• make fmt, make vet, make golangci, make test (CI)
• Sequential e2e: prometheus_metrics and opentelemetry_metrics

[snyk-io]

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[Copilot]

Pull request overview

This PR adds performance-focused metrics around KEDA’s “metric server” flow: HPA external metrics requests handled by keda-metrics-apiserver and GetMetrics gRPC calls handled by keda-operator, with both Prometheus and optional OpenTelemetry coverage.

Changes:

• Instrument GetExternalMetric (adapter) and GetMetrics (operator) with request-count and duration metrics (Prometheus + optional OTEL).
• Add unit tests for the new metrics collectors and extend sequential e2e metric assertions for both Prometheus and OpenTelemetry scenarios.
• Document the user-visible change in CHANGELOG.md.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/sequential/prometheus_metrics/prometheus_metrics_test.go	Adds assertions for new Prometheus performance metrics (adapter + operator).
tests/sequential/opentelemetry_metrics/opentelemetry_metrics_test.go	Adds assertions for OpenTelemetry-exported (Prometheus-exposed) operator GetMetrics metrics.
pkg/provider/provider.go	Instruments adapter external metrics requests and records duration/outcome.
pkg/metricsservice/server.go	Instruments operator GetMetrics gRPC handler and records duration/outcome.
pkg/metricscollector/prommetrics.go	Defines/registers new Prometheus metrics and records GetMetrics request metrics in operator collector.
pkg/metricscollector/opentelemetry.go	Defines OTEL instruments and records GetMetrics request metrics in operator OTEL collector.
pkg/metricscollector/opentelemetry_test.go	Adds unit test coverage for the new OTEL GetMetrics instruments.
pkg/metricscollector/metricserver_performance.go	Introduces adapter-side performance metrics (Prometheus + optional OTEL exporter init).
pkg/metricscollector/metricserver_performance_test.go	Adds unit tests for adapter performance metrics and shared request-result labeling.
pkg/metricscollector/metricscollectors.go	Extends the collector interface and adds a package-level recording helper for operator GetMetrics.
pkg/metricscollector/export_test.go	Exposes test helpers for adapter OTEL performance metrics initialization/reset.
cmd/adapter/main.go	Registers adapter performance metrics and adds --enable-opentelemetry-metrics flag.
CHANGELOG.md	Adds an “Unreleased” Improvements entry for the new performance metrics.

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[JorTurFer]

/run-e2e sequential
Update: You can check the progress here

passed tests: 5

Execution of tests/sequential/prometheus_metrics/prometheus_metrics_test.go, has passed after "one" attempts
Execution of tests/sequential/disruption/disruption_test.go, has passed after "one" attempts
Execution of tests/sequential/datadog_dca/datadog_dca_test.go, has passed after "one" attempts
Execution of tests/sequential/opentelemetry_metrics/opentelemetry_metrics_test.go, has passed after "one" attempts
Execution of tests/sequential/broken_scaledobject_tolerancy/broken_scaledobject_tolerancy_test.go, has passed after "one" attempts

failed tests: 0

[rickbrouwer]

/run-e2e sequential
Update: You can check the progress here

passed tests: 5

Execution of tests/sequential/prometheus_metrics/prometheus_metrics_test.go, has passed after "one" attempts
Execution of tests/sequential/broken_scaledobject_tolerancy/broken_scaledobject_tolerancy_test.go, has passed after "one" attempts
Execution of tests/sequential/opentelemetry_metrics/opentelemetry_metrics_test.go, has passed after "one" attempts
Execution of tests/sequential/datadog_dca/datadog_dca_test.go, has passed after "one" attempts
Execution of tests/sequential/disruption/disruption_test.go, has passed after "one" attempts

failed tests: 0

[kshitizlohia1994]

@rickbrouwer @JorTurFer can you please review when you get a chance?