Component(s)

target allocator

Is your feature request related to a problem? Please describe.

Problem

The opentelemetry_allocator_targets_remaining metric currently reports a scalar count of unallocated targets, making it impossible to distinguish
between:

1. Intentionally filtered targets (e.g., namespaces excluded via allowNamespaces)
2. Orphaned targets due to bugs (e.g., stale collector UIDs, version skew during rollouts)

This makes alerting extremely difficult - we can't set meaningful thresholds without either:

• Creating false positives (alerting on intentional filters)
• Missing real issues (high threshold that ignores single critical target losses)

Use Case

In our production environment:

• We filter istio-system namespace targets (~1600 targets) to control costs
• This baseline makes targets_remaining unusable for detecting orphaned targets
• A recent incident (target assigned to deleted collector pod) went undetected for 14 hours

Describe the solution you'd like

Proposed Solution

Add labels to opentelemetry_allocator_targets_remaining to break down unallocated targets:

targetsRemaining = promauto.NewGaugeVec(
    prometheus.GaugeOpts{
        Name: "opentelemetry_allocator_targets_remaining",
        Help: "Number of targets that could not be allocated to any collector",
    },
    []string{"job_name", "namespace"},  // Add these labels
)

This would allow alerts like:

  # Alert on orphaned targets, excluding known filtered namespaces
  opentelemetry_allocator_targets_remaining{namespace!="istio-system"} > 10

Alternative

If per-target labeling is too expensive (high cardinality), consider:

• Add just reason label: {reason="namespace_filtered"} vs {reason="no_collectors"}
• Or expose separate metrics: ..._targets_filtered vs ..._targets_orphaned

Describe alternatives you've considered

No response

Additional context

No response

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