Provide support for metrics through OpenTelemetry

[tomkerkhove]

Proposal

Provide support for metrics through OpenTelemetry which allows end-users to use the destination that they prefer.

Source: https://opentelemetry.io/docs/

Ideally, end-users can use the OTLP collector and take it from there.

Use-Case

Allows end-users to use the destination that they prefer instead of us having to implement every one of them.

We already support Prometheus, and HTTP add-on is adding support through kedacore/http-add-on#321, but this allows us to be more flexible.

Anything else?

Metrics in OpenTelemetry spec has just gone stable (https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/api.md) but some pieces are still experimental so we should wait (https://github.com/open-telemetry/opentelemetry-specification/tree/main/specification/metrics)

Go support is in Alpha (https://opentelemetry.io/docs/go/#status-and-releases) but already has decent coverage (https://github.com/open-telemetry/opentelemetry-specification/blob/main/spec-compliance-matrix.md#metrics)

Long story short, already tracking for when it goes stable.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions.

[tomkerkhove]

Duplicate of #3078