test(target-allocator): guard scrape_protocols defaulting on config load

Add a regression test asserting that loading a Target Allocator config whose
static scrape job omits scrape_protocols still yields a non-empty
ScrapeProtocols on every loaded scrape config. This is defaulted by the pinned
Prometheus library during yaml.UnmarshalStrict into the prometheus Config type,
so the distributed /scrape_configs payload is never empty and the agent's
prometheus-receiver validation passes. The test fails fast if a future
dependency or load-path change drops this defaulting.

Author: musa-asad

cmd/amazon-cloudwatch-agent-target-allocator/config/scrape_protocols_regression_test.go

@@ -0,0 +1,36 @@
+// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package config
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestScrapeProtocolsDefaultedOnLoad guards against "scrape_protocols cannot be empty" regression.
+func TestScrapeProtocolsDefaultedOnLoad(t *testing.T) {
+	got := CreateDefaultConfig()
+	err := LoadFromFile("./testdata/scrape_protocols_omitted_test.yaml", &got)
+	require.NoError(t, err)
+
+	require.NotNil(t, got.PromConfig)
+	require.NotEmpty(t, got.PromConfig.ScrapeConfigs)
+
+	for _, sc := range got.PromConfig.ScrapeConfigs {
+		assert.NotEmpty(t, sc.ScrapeProtocols,
+			"scrape_protocols must be defaulted for job %q", sc.JobName)
+	}
+
+	// Spot-check the specific static job from the reproduction by name.
+	found := false
+	for _, sc := range got.PromConfig.ScrapeConfigs {
+		if sc.JobName == "prometheus-sample-app" {
+			found = true
+			assert.Greater(t, len(sc.ScrapeProtocols), 0)
+		}
+	}
+	require.True(t, found, "expected the 'prometheus-sample-app' job to be present")
+}

cmd/amazon-cloudwatch-agent-target-allocator/config/testdata/scrape_protocols_omitted_test.yaml

@@ -0,0 +1,17 @@
+# Regression fixture for "scrape_protocols cannot be empty".
+# A static scrape job that deliberately OMITS scrape_protocols, mirroring the
+# quip-doc "prometheus-sample-app" Test 5 job. Loading this through the TA config
+# path must default scrape_protocols onto every scrape config (Prometheus
+# v0.311.3 ScrapeConfig.UnmarshalYAML), so the distributed config is never empty
+# and the agent's prometheus-receiver validation passes.
+config:
+  scrape_configs:
+    - job_name: prometheus-sample-app
+      kubernetes_sd_configs:
+        - role: pod
+      relabel_configs:
+        - source_labels: [__meta_kubernetes_pod_label_app]
+          regex: prometheus-sample-app
+          action: keep
+        - target_label: label1
+          replacement: value1