add missing unit test, fix inconsistency in reported error

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

config/metric.go

@@ -100,7 +100,7 @@ func periodicExporter(ctx context.Context, exporter MetricExporter, opts ...sdkm
 		case protocolProtobufGRPC:
 			exp, err = otlpGRPCMetricExporter(ctx, exporter.OTLP)
 		default:
-			return nil, fmt.Errorf("unsupported protocol %s", exporter.OTLP.Protocol)
+			return nil, fmt.Errorf("unsupported protocol %q", exporter.OTLP.Protocol)
 		}
 		if err != nil {
 			return nil, err

config/metric_test.go

@@ -153,6 +153,19 @@ func TestReader(t *testing.T) {
 			},
 			wantReader: readerWithServer{promExporter, nil},
 		},
+		{
+			name: "periodic/otlp-exporter-invalid-protocol",
+			reader: MetricReader{
+				Periodic: &PeriodicMetricReader{
+					Exporter: MetricExporter{
+						OTLP: &OTLPMetric{
+							Protocol: "http/invalid",
+						},
+					},
+				},
+			},
+			wantErr: errors.New("unsupported protocol \"http/invalid\""),
+		},
 		{
 			name: "periodic/otlp-grpc-exporter",
 			reader: MetricReader{

config/trace_test.go

@@ -209,7 +209,7 @@ func TestSpanProcessor(t *testing.T) {
 					ScheduleDelay:      ptr(0),
 					Exporter: SpanExporter{
 						OTLP: &OTLP{
-							Protocol: *ptr("http/invalid"),
+							Protocol: "http/invalid",
 						},
 					},
 				},