feat: allow flush level OFF (#319)

Author: daniel-sanche

src/main/java/com/google/cloud/logging/logback/LoggingAppender.java

@@ -211,7 +211,10 @@ public synchronized void start() {
     MonitoredResource resource = getMonitoredResource(getProjectId());
     defaultWriteOptions =
         new WriteOption[] {WriteOption.logName(getLogName()), WriteOption.resource(resource)};
-    getLogging().setFlushSeverity(severityFor(getFlushLevel()));
+    Level flushLevel = getFlushLevel();
+    if (flushLevel != Level.OFF) {
+      getLogging().setFlushSeverity(severityFor(flushLevel));
+    }
     loggingEnhancers = new ArrayList<>();
     List<LoggingEnhancer> resourceEnhancers = MonitoredResourceUtil.getResourceEnhancers();
     loggingEnhancers.addAll(resourceEnhancers);

src/test/java/com/google/cloud/logging/logback/LoggingAppenderTest.java

@@ -112,6 +112,14 @@ public void testFlushLevelConfigUpdatesLoggingFlushSeverity() {
     assertThat(capturedArgument.getValue().iterator().next()).isEqualTo(logEntry);
   }
 
+  @Test
+  public void testFlushLevelConfigSupportsFlushLevelOff() {
+    loggingAppender.setFlushLevel(Level.OFF);
+    loggingAppender.start();
+    Severity foundSeverity = logging.getFlushSeverity();
+    assertThat(foundSeverity).isEqualTo(null);
+  }
+
   @Test
   public void testFilterLogsOnlyLogsAtOrAboveLogLevel() {
     Map<String, Object> jsonContent = new HashMap<>();