config: fix bug where WithResourceAsConstantLabels wasn't set

There was a bug in the config package where the WithResourceAsConstantLabels
option wasn't appended to the list of options for the Prometheus reader. Fixed
that issue and added a test to validate the correct number of options is
set.

Signed-off-by: Alex Boten <223565+codeboten@users.noreply.github.com>

config/metric.go

@@ -297,14 +297,8 @@ func newIncludeExcludeFilter(lists *IncludeExclude) (attribute.Filter, error) {
 	}, nil
 }
 
-func prometheusReader(ctx context.Context, prometheusConfig *Prometheus) (sdkmetric.Reader, error) {
+func prometheusReaderOpts(prometheusConfig *Prometheus) ([]otelprom.Option, error) {
 	var opts []otelprom.Option
-	if prometheusConfig.Host == nil {
-		return nil, fmt.Errorf("host must be specified")
-	}
-	if prometheusConfig.Port == nil {
-		return nil, fmt.Errorf("port must be specified")
-	}
 	if prometheusConfig.WithoutScopeInfo != nil && *prometheusConfig.WithoutScopeInfo {
 		opts = append(opts, otelprom.WithoutScopeInfo())
 	}
@@ -319,7 +313,23 @@ func prometheusReader(ctx context.Context, prometheusConfig *Prometheus) (sdkmet
 		if err != nil {
 			return nil, err
 		}
-		otelprom.WithResourceAsConstantLabels(f)
+		opts = append(opts, otelprom.WithResourceAsConstantLabels(f))
+	}
+
+	return opts, nil
+}
+
+func prometheusReader(ctx context.Context, prometheusConfig *Prometheus) (sdkmetric.Reader, error) {
+	if prometheusConfig.Host == nil {
+		return nil, fmt.Errorf("host must be specified")
+	}
+	if prometheusConfig.Port == nil {
+		return nil, fmt.Errorf("port must be specified")
+	}
+
+	opts, err := prometheusReaderOpts(prometheusConfig)
+	if err != nil {
+		return nil, err
 	}
 
 	reg := prometheus.NewRegistry()

config/metric_test.go

@@ -1138,3 +1138,44 @@ func TestNewIncludeExcludeFilterError(t *testing.T) {
 	}))
 	require.Equal(t, fmt.Errorf("attribute cannot be in both include and exclude list: foo"), err)
 }
+
+func TestPrometheusReaderOpts(t *testing.T) {
+	testCases := []struct {
+		name        string
+		cfg         Prometheus
+		wantOptions int
+	}{
+		{
+			name:        "no options",
+			cfg:         Prometheus{},
+			wantOptions: 0,
+		},
+		{
+			name: "all set",
+			cfg: Prometheus{
+				WithoutScopeInfo:           ptr(true),
+				WithoutTypeSuffix:          ptr(true),
+				WithoutUnits:               ptr(true),
+				WithResourceConstantLabels: &IncludeExclude{},
+			},
+			wantOptions: 4,
+		},
+		{
+			name: "all set false",
+			cfg: Prometheus{
+				WithoutScopeInfo:           ptr(false),
+				WithoutTypeSuffix:          ptr(false),
+				WithoutUnits:               ptr(false),
+				WithResourceConstantLabels: &IncludeExclude{},
+			},
+			wantOptions: 1,
+		},
+	}
+	for _, tt := range testCases {
+		t.Run(tt.name, func(t *testing.T) {
+			opts, err := prometheusReaderOpts(&tt.cfg)
+			require.NoError(t, err)
+			require.Len(t, opts, tt.wantOptions)
+		})
+	}
+}