[receiver/purefa] Fix the receiver failing to start due to an invalid internal Prometheus scrape configuration (#48850)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description

The receiver used a `*discovery.StaticConfig` pointer when building the
Prometheus scrape configs. Only the value type `discovery.StaticConfig`
is registered for YAML marshaling, so the prometheus receiver failed on
startup with `cannot marshal unregistered Config type:
*discovery.StaticConfig`. The config now uses the value type.

<!-- Issue number (e.g. #1234) or full URL to issue, if applicable. -->
#### Link to tracking issue
Fixes #48847

<!--Describe what testing was performed and which tests were added.-->
#### Testing

Added unit tests to cover this issue.

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#### Authorship

- [X] I, a human, wrote this pull request description myself.

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---------

Signed-off-by: Paulo Dias <paulodias.gm@gmail.com>

Author: paulojmdias

.chloggen/48847-purefareceiver-static-config-marshal.yaml

@@ -0,0 +1,10 @@
+change_type: bug_fix
+component: receiver/purefa
+note: Fix the receiver failing to start due to an invalid internal Prometheus scrape configuration.
+issues: [48847]
+subtext: |
+  The receiver used a `*discovery.StaticConfig` pointer when building the Prometheus scrape
+  configs. Only the value type `discovery.StaticConfig` is registered for YAML marshaling, so
+  the prometheus receiver failed on startup with "cannot marshal unregistered Config type:
+  *discovery.StaticConfig". The config now uses the value type.
+change_logs: [user]

receiver/purefareceiver/go.mod

@@ -23,6 +23,7 @@ require (
 	go.opentelemetry.io/collector/receiver/receivertest v0.154.1-0.20260611154946-8ae0933981eb
 	go.uber.org/goleak v1.3.0
 	go.uber.org/multierr v1.11.0
+	gopkg.in/yaml.v2 v2.4.0
 )
 
 require (
@@ -260,7 +261,6 @@ require (
 	gopkg.in/evanphx/json-patch.v4 v4.13.0 // indirect
 	gopkg.in/inf.v0 v0.9.1 // indirect
 	gopkg.in/ini.v1 v1.67.2 // indirect
-	gopkg.in/yaml.v2 v2.4.0 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 	k8s.io/api v0.35.3 // indirect
 	k8s.io/apimachinery v0.35.3 // indirect

receiver/purefareceiver/internal/scraper.go

@@ -101,7 +101,10 @@ func (h *scraper) ToPrometheusReceiverConfig(host component.Host, _ receiver.Fac
 			},
 
 			ServiceDiscoveryConfigs: discovery.Configs{
-				&discovery.StaticConfig{
+				// Must be a value, not a pointer: only discovery.StaticConfig is
+				// registered for YAML marshaling, which the prometheus receiver
+				// performs on startup.
+				discovery.StaticConfig{
 					{
 						Targets: []model.LabelSet{
 							{model.AddressLabel: model.LabelValue(u.Host)},

receiver/purefareceiver/internal/scraper_test.go

@@ -14,6 +14,7 @@ import (
 	"go.opentelemetry.io/collector/config/configauth"
 	"go.opentelemetry.io/collector/config/configtls"
 	"go.opentelemetry.io/collector/extension/extensiontest"
+	"gopkg.in/yaml.v2"
 
 	"github.com/open-telemetry/opentelemetry-collector-contrib/extension/bearertokenauthextension"
 	"github.com/open-telemetry/opentelemetry-collector-contrib/receiver/prometheusreceiver"
@@ -71,3 +72,42 @@ func TestToPrometheusConfig(t *testing.T) {
 	assert.EqualValues(t, interval, scCfgs[0].ScrapeTimeout)
 	assert.EqualValues(t, interval, scCfgs[0].ScrapeInterval)
 }
+
+// TestToPrometheusConfigMarshalsToYAML guards against using a *discovery.StaticConfig
+// pointer, which is unregistered and fails the YAML marshal the prometheus receiver
+// runs on startup.
+func TestToPrometheusConfigMarshalsToYAML(t *testing.T) {
+	prFactory := prometheusreceiver.NewFactory()
+	baFactory := bearertokenauthextension.NewFactory()
+
+	baCfg := baFactory.CreateDefaultConfig().(*bearertokenauthextension.Config)
+	baCfg.BearerToken = "the-token"
+
+	baExt, err := baFactory.Create(t.Context(), extensiontest.NewNopSettings(baFactory.Type()), baCfg)
+	require.NoError(t, err)
+
+	host := &mockHost{
+		extensions: map[component.ID]component.Component{
+			component.MustNewIDWithName("bearertokenauth", "array01"): baExt,
+		},
+	}
+
+	cfgs := []ScraperConfig{
+		{
+			Address: "array01",
+			Auth: configauth.Config{
+				AuthenticatorID: component.MustNewIDWithName("bearertokenauth", "array01"),
+			},
+		},
+	}
+
+	scraper := NewScraper(t.Context(), "hosts", "http://example.com", "purefa", configtls.ClientConfig{}, cfgs, 15*time.Second, model.LabelSet{})
+
+	scCfgs, err := scraper.ToPrometheusReceiverConfig(host, prFactory)
+	require.NoError(t, err)
+	require.Len(t, scCfgs, 1)
+
+	// Mirrors the marshaling the prometheus receiver performs on startup.
+	_, err = yaml.Marshal(scCfgs[0])
+	require.NoError(t, err)
+}