feat(scaler): add observability (metrics + tracing) to the external scaler

[Fedosin]

Add OpenTelemetry-based metrics and distributed tracing to the external
scaler component, which previously had no observability instrumentation.

Metrics:

• scaler.pinger.fetch.duration (histogram) — duration of each queue pinger fetch cycle
• scaler.pinger.fetch.errors (counter) — total failed pinger fetch cycles
• scaler.pinger.endpoints (gauge) — number of interceptor endpoints being polled
• Prometheus /metrics endpoint on port 2224 (configurable via OTEL_PROM_EXPORTER_PORT)
• Optional OTLP HTTP metrics export (via OTEL_EXPORTER_OTLP_METRICS_ENABLED)

Tracing:

• OTEL tracing SDK with console, HTTP/protobuf, and gRPC exporters
• otelgrpc stats handler for automatic gRPC server span instrumentation
• W3C TraceContext + Baggage propagation

Configuration env vars (same convention as the interceptor):

• OTEL_PROM_EXPORTER_ENABLED (default: true)
• OTEL_PROM_EXPORTER_PORT (default: 2224)
• OTEL_EXPORTER_OTLP_METRICS_ENABLED (default: false)
• OTEL_EXPORTER_OTLP_TRACES_ENABLED (default: false)
• OTEL_EXPORTER_OTLP_TRACES_PROTOCOL (default: console)

Checklist

• Commits are signed with Developer Certificate of Origin (DCO)
• Changelog has been updated and is aligned with our changelog requirements
• Any necessary documentation is added, such as:
  • README.md
  • keda-docs

Fixes #965 (partial — scaler component)

[snyk-io]

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[Copilot]

Pull request overview

This PR adds OpenTelemetry-based observability to the external scaler by introducing metric instruments/exporters and distributed tracing, plus wiring them into the scaler’s startup and queue polling logic.

Changes:

• Added an OTEL metrics provider + instruments for queue pinger fetch duration/errors and endpoint count, with Prometheus and optional OTLP/HTTP export.
• Added OTEL tracing SDK setup and enabled automatic gRPC server span instrumentation via otelgrpc when tracing is enabled.
• Wired metrics/tracing configuration into scaler config and main startup, including a /metrics HTTP endpoint.

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
scaler/tracing/tracing.go	Adds OTEL tracing SDK setup and exporter selection for the scaler.
scaler/metrics/provider.go	Introduces an OTEL MeterProvider with Prometheus and optional OTLP metric export.
scaler/metrics/instruments.go	Defines metric instruments and recording helpers for queue pinger metrics.
scaler/queue_pinger.go	Records pinger fetch metrics on each polling cycle.
scaler/queue_pinger_test.go	Updates pinger construction in tests for the new instruments parameter.
scaler/main.go	Initializes metrics/tracing, adds Prometheus /metrics server, and instruments gRPC when enabled.
scaler/config.go	Adds env-configurable metrics and tracing settings for the scaler.
go.mod	Adds otelgrpc dependency for gRPC tracing instrumentation.
go.sum	Updates sums for added/updated dependencies.

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