fix MissingCasesInEnumSwitch (#5595)

Non-exhaustive switch; either add a default or handle the remaining cases: VALUE, UNKNOWN
Co-authored-by: 吴晟 Wu Sheng <wu.sheng@foxmail.com>

apm-application-toolkit/apm-toolkit-micrometer-registry/src/main/java/org/apache/skywalking/apm/meter/micrometer/SkywalkingMeterRegistry.java

@@ -126,6 +126,8 @@ public class SkywalkingMeterRegistry extends MeterRegistry {
                 case DURATION:
                     meterName = baseName + "_duration_sum";
                     break;
+                default:
+                    break;
             }
 
             if (isCounter) {