feat: Add control plane telemetry - scheduler eligible workers histogram

[Angela (Angelawork)]

Part of #564 (Part 3)

• Tests pass
• 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/...

[Jeff Luo (JeffLuoo)]

Please rebase and fix e2e tests

[Jeff Luo (JeffLuoo)]

Please do not use Fixes #564 (Part 3) in the description as it will close the bug. Use Part of #564 (Part 3)

[Jeff Luo (JeffLuoo)]

For some reasons the CI is not running.. I close and reopen the PR to see if it will trigger the presubmit again.