[gateway] Add Prometheus identity labels

[platinumhamburg]

Purpose

Linked issue: close #4217

Brief change log

Tests

API and Format

Documentation

[beryllw]

My main question is about the Gateway identity model. We now have:

• gateway.id: logical Gateway deployment
• gateway.instance-id: individual Gateway instance used for diagnostics
• gateway.host: host or network location

Is gateway.host expected to uniquely identify each Gateway replica? If it only represents location, multiple replicas may share the same value. In that case, could we expose the existing gateway.instance-id as the per-replica metric label instead?

[beryllw]

Could we add an integration test that starts the Gateway with gateway.id and gateway.host, then verifies the labels from the actual /metrics endpoint? It would cover the configuration, lifecycle, and exporter wiring together.

[platinumhamburg]

My main question is about the Gateway identity model. We now have:

• gateway.id: logical Gateway deployment
• gateway.instance-id: individual Gateway instance used for diagnostics
• gateway.host: host or network location

Is gateway.host expected to uniquely identify each Gateway replica? If it only represents location, multiple replicas may share the same value. In that case, could we expose the existing gateway.instance-id as the per-replica metric label instead?

Good point. I removed gateway.host and reused the existing gateway.instance-id as the per-replica identity. It is now exposed as the instance_id metric label, while gateway.id remains the identity shared by one logical Gateway deployment. This avoids introducing two overlapping per-replica identity configurations.

[platinumhamburg]

Could we add an integration test that starts the Gateway with gateway.id and gateway.host, then verifies the labels from the actual /metrics endpoint? It would cover the configuration, lifecycle, and exporter wiring together.

Added an end-to-end HTTP test that starts the Gateway with gateway.id and gateway.instance-id, fetches the actual /metrics endpoint, and verifies the gateway_id and instance_id labels. The existing configuration tests continue to cover YAML and environment-variable loading.