Angela 058b104dec feat: Add control plane telemetry - scheduler eligible workers histogram (#682) ...

Part of #564 (Part 3)

- [x] Tests pass
- [x] Appropriate changes to documentation are included in the PR

## Description
This PR implements Part 3 of #564 by adding telemetry histogram
instrumentation for `ate.scheduler.eligible_workers` in `ateapi`. It
measures unassigned free worker capacity remaining after all scheduling
constraint filters are applied, sampled at every scheduling decision.

## Key Changes:
* Updated `Scheduler.Schedule()` to record eligible candidate workers
per pool (`recordEligibleWorkers`).
Defined `SchedulingConstraintKey` (`ate.scheduling.constraint`) and
constraint classification values (`none`, `required_nodes`, `selector`)
in `internal/ateattr/ateattr.go`.
* Added unit tests in `scheduling_test.go` covering candidate counts,
namespaced attributes, zero-capacity fleet states, empty fleets, sandbox
class mismatches, draining workers, and constraint classifications.

## Testing
* `go test -buildvcs=false ./cmd/ateapi/internal/controlapi/...`
* `go test -buildvcs=false ./cmd/atenet/internal/router/...`
* `make test`
## E2E  Test
* `./hack/create-kind-cluster.sh`
* `./hack/install-ate-kind.sh --deploy-ate-system --deploy-demo-counter`
* `./hack/run-e2e.sh ./internal/e2e/suites/metrics/...`

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..

metrics.go

feat: Add control plane telemetry - scheduler eligible workers histogram (#682)

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scheduling_test.go

feat: Add control plane telemetry - scheduler eligible workers histogram (#682)

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scheduling.go

feat: Add control plane telemetry - scheduler eligible workers histogram (#682)

2026-08-07 09:29:20 -04:00