feat: add support for additional labels on metrics

[rajsameer]

Add generic --extra-labels flag for customising metric labels

Summary

Introduces a new --extra-labels CLI flag that allows operators to attach additional labels to any metric across all OpenStack service exporters, without requiring code changes. This
replaces the narrower, Ironic-specific --ironic.additional-labels approach with a generic solution.

Motivation

Users often need to enrich metrics with environment-specific context (e.g. rack ID, datacenter, environment name) that is available in OpenStack API responses but not exposed as
Prometheus labels by default. Previously, adding such labels required service-specific code changes per exporter.

Changes

New --extra-labels flag (main.go, utils/kingping_extra_labels_parser.go)

• Accepts one or more --extra-labels=.: arguments (cumulative flag)
• Supports two label types per invocation:
  • Dynamic labels — resolved from API response fields via dot-path (e.g. conductor, extra.rack_id → label name extra_rack_id)
  • Static/const labels — fixed key=value pairs appended to every metric sample (e.g. env=production)
• Full validation: Prometheus label name constraints enforced, duplicates rejected
• Example: --extra-labels=ironic.node:conductor,extra.rack_id,datacenter=dc1

Generic reflect-based field resolver (exporters/fields_cahce.go)

• resolveField(obj, "extra.rack_id") — resolves dot-paths against any struct using a JSON-tag-keyed index; supports nested map[string]interface{} access
• getFieldIndex — lazily built, thread-safe cache (sync.RWMutex with double-checked locking) keyed by reflect.Type, shared globally across all exporters
• computeMetricLabels / computeConstantLabels — helpers used by all exporter constructors to append extra variable/const labels to metric descriptors

All service exporters updated

Extra label support added to all 15 exporters: ironic, cinder, nova, neutron, glance, keystone, loadbalancer, heat, placement, manila, designate, trove, gnocchi, containerinfra,
objectstore

Testing

• utils/kingpin_parser_test.go — 10 new tests covering parsing, dot-path conversion, static labels, cumulative behaviour, error cases, and Extract
• exporters/fields_cache_test.go — 21 new tests covering resolveField, computeMetricLabels, computeConstantLabels, and resolveExtraLabelValues
• exporters/ironic_test.go — End-to-end integration test: creates an Ironic exporter with ironic.node:conductor,env=test and validates that all 5 fixture nodes carry both the dynamic
  conductor label and the static env="test" const label in the collected output

Usage

Add the conductor field and a static datacenter label to all ironic node metrics

openstack-exporter --extra-labels=ironic.node:conductor,datacenter=dc1 mycloud

Add a nested extra field and a static env label to ironic node metrics

openstack-exporter --extra-labels=ironic.node:extra.rack_id,env=production mycloud

Multiple services in one invocation (cumulative flag)

openstack-exporter
--extra-labels=ironic.node:conductor
--extra-labels=nova.server:hypervisor_hostname,region=eu-west
mycloud

[Sharpz7]

This is really cool! I had envisioned this in #386.

I wonder if this can be extended more widely

[rajsameer]

@Sharpz7 instead of additional labels how about overriding labels. Let me take some time and implement it for all metrics.
exporter users will then have a way to add or remove labels customizing their metrics

[rajsameer]

@Sharpz7 can you review the PR

[Sharpz7]

Hey @rajsameer. I will try to get to it ASAP

[Sharpz7]

@rajsameer are you able to provide some results from a devstack with this working?

[rajsameer]

@rajsameer are you able to provide some results from a devstack with this working?

I am using the exporter for ironic metrics, I do not have all components of openstack. Do you need the metrics output?

[Sharpz7]

Its more that I would like this tested before we merge it. In a devstack, or potentially integration tests when #481 lands.

I appreciate you have already put a lot of work in, so I'll try my best to help out getting this merged.

[rajsameer]

Its more that I would like this tested before we merge it. In a devstack, or potentially integration tests when #481 lands.

I appreciate you have already put a lot of work in, so I'll try my best to help out getting this merged.

hey sorry for the late reply did not get time from work. I will try to get the verification done in few days.