target relabel does not work in otel-allocator

[hanjintao1996]

Component(s)

target allocator

What happened?

Title: Bug in Apply Method: Modified lset Not Assigned Back to tItem.Labels

Description:
There is a bug in the Apply method of relabelConfigTargetFilter in relabel.go where the modified label set (lset) is not being assigned back to tItem.Labels. This results in any changes made during the relabeling process not being reflected in the target items.
File: cmd/otel-allocator/internal/prehook/relabel.go
In the code, if the target is not dropped, the modified labels will not be assigned to tItem.

func (tf *relabelConfigTargetFilter) Apply(targets []*target.Item) []*target.Item {
	numTargets := len(targets)

	// need to wait until relabelCfg is set
	if len(tf.relabelCfg) == 0 {
		return targets
	}

	writeIndex := 0
	for _, tItem := range targets {
		keepTarget := true
		lset := tItem.Labels
		for _, cfg := range tf.relabelCfg[tItem.JobName] {
			lset, keepTarget = relabel.Process(lset, cfg)
			if !keepTarget {
				break // inner loop
			}
		}

		if keepTarget {
			targets[writeIndex] = tItem
			writeIndex++
		}
	}

	targets = targets[:writeIndex]
	targets = slices.Clip(targets)
	tf.log.V(2).Info("Filtering complete", "seen", numTargets, "kept", len(targets))
	return targets
}

Steps to Reproduce:

Use a relabel configuration that modifies the label set.
Call the Apply method.
Observe that changes in labels are not reflected in the target items.
Expected Behavior:
The modified label set (lset) should be assigned back to tItem.Labels to ensure that any changes made during the relabeling process are reflected.

Actual Behavior:
The modified lset is not assigned back, resulting in no changes to the target items’ labels after processing.

Proposed Solution:
Assign the modified lset back to tItem.Labels within the keepTarget block:

if keepTarget {
    tItem.Labels = lset // Assign processed label set back to tItem
    tItem.TargetURL = tItem.Labels.Get("__address__")  // Assign processed label set back to tItem
    targets[writeIndex] = tItem
    writeIndex++
}

Please let me know if you need any further details.

Kubernetes Version

1.23.0

Operator version

v0.4.0

Collector version

v0.4.0

Environment information

Environment

OS: (e.g., "Ubuntu 20.04")
Compiler(if manually compiled): (e.g., "go 14.2")

Log output

Additional context

No response

[swiatekm]

This is expected behavior. The target allocator only removes targets based on the relabel rules, it does not modify the labels, but rather leaves that to the prometheus receiver. There's been some effort in the past to make it work the way you expected, but that requires mirroring the way Prometheus initializes labels in general, and is quite complex. See #4074 for a relatively small piece of the work needed.