Annotate MetadataKey.emit... methods to indicate that the value can be null. Fix implementations to account for null values (by ignoring nulls).

api/src/main/java/com/google/common/flogger/LogContext.java

@@ -106,7 +106,7 @@ private Key() {}
     public static final MetadataKey<Object> LOG_SITE_GROUPING_KEY =
         new MetadataKey<Object>("group_by", Object.class, true) {
           @Override
-          public void emitRepeated(Iterator<Object> keys, KeyValueHandler out) {
+          public void emitRepeated(Iterator<?> keys, KeyValueHandler out) {
             if (keys.hasNext()) {
               Object first = keys.next();
               if (!keys.hasNext()) {
@@ -175,7 +175,10 @@ public void emitRepeated(Iterator<Object> keys, KeyValueHandler out) {
     public static final MetadataKey<Tags> TAGS =
         new MetadataKey<Tags>("tags", Tags.class, false) {
           @Override
-          public void emit(Tags tags, KeyValueHandler out) {
+          public void emit(@Nullable Tags tags, KeyValueHandler out) {
+            if (tags == null) {
+              return;
+            }
             for (Map.Entry<String, ? extends Set<Object>> e : tags.asMap().entrySet()) {
               Set<Object> values = e.getValue();
               if (!values.isEmpty()) {

api/src/main/java/com/google/common/flogger/MetadataKey.java

@@ -171,7 +171,7 @@ public final boolean canRepeat() {
    * Emits one or more key/value pairs for the given metadata value. Call this method in preference
    * to using {@link #emit} directly to protect against unbounded reentrant logging.
    */
-  public final void safeEmit(T value, KeyValueHandler kvh) {
+  public final void safeEmit(@Nullable T value, KeyValueHandler kvh) {
     if (isCustom && Platform.getCurrentRecursionDepth() > MAX_CUSTOM_METADATAKEY_RECURSION_DEPTH) {
       // Recursive logging detected, possibly caused by custom metadata keys triggering reentrant
       // logging. To halt recursion, emit the keys in the default non-custom format without invoking
@@ -187,7 +187,7 @@ public final void safeEmit(T value, KeyValueHandler kvh) {
    * in preference to using {@link #emitRepeated} directly to protect against unbounded reentrant
    * logging.
    */
-  public final void safeEmitRepeated(Iterator<T> values, KeyValueHandler kvh) {
+  public final void safeEmitRepeated(Iterator<? extends @Nullable T> values, KeyValueHandler kvh) {
     checkState(canRepeat, "non repeating key");
     if (isCustom && Platform.getCurrentRecursionDepth() > MAX_CUSTOM_METADATAKEY_RECURSION_DEPTH) {
       // Recursive logging detected, possibly caused by custom metadata keys triggering reentrant
@@ -236,7 +236,7 @@ public final void safeEmitRepeated(Iterator<T> values, KeyValueHandler kvh) {
    *
    * <p>By default this method just calls {@code out.handle(getLabel(), value)}.
    */
-  protected void emit(T value, KeyValueHandler kvh) {
+  protected void emit(@Nullable T value, KeyValueHandler kvh) {
     kvh.handle(getLabel(), value);
   }
 
@@ -253,7 +253,7 @@ protected void emit(T value, KeyValueHandler kvh) {
    * <p>See the {@link #emit(Object,KeyValueHandler)} method for additional caveats for custom
    * implementations.
    */
-  protected void emitRepeated(Iterator<T> values, KeyValueHandler kvh) {
+  protected void emitRepeated(Iterator<? extends @Nullable T> values, KeyValueHandler kvh) {
     while (values.hasNext()) {
       emit(values.next(), kvh);
     }

api/src/test/java/com/google/common/flogger/MetadataKeyTest.java

@@ -51,7 +51,7 @@ protected void emit(Object value, KeyValueHandler kvh) {
         }
 
         @Override
-        protected void emitRepeated(Iterator<Object> values, KeyValueHandler kvh) {
+        protected void emitRepeated(Iterator<?> values, KeyValueHandler kvh) {
           // Hack for test to preserve the given values past a single use. In normal logging there
           // would be a new Metadata instance created for each of the reentrant logging calls.
           ImmutableList<Object> copy = ImmutableList.copyOf(values);

api/src/test/java/com/google/common/flogger/backend/system/SimpleLogRecordTest.java

@@ -31,6 +31,7 @@
 import java.util.Iterator;
 import java.util.logging.Level;
 import java.util.logging.LogRecord;
+import org.jspecify.annotations.Nullable;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
@@ -42,7 +43,7 @@ public class SimpleLogRecordTest {
   private static final MetadataKey<String> PATH_KEY =
       new MetadataKey<String>("path", String.class, true) {
         @Override
-        public void emitRepeated(Iterator<String> values, KeyValueHandler out) {
+        public void emitRepeated(Iterator<? extends @Nullable String> values, KeyValueHandler out) {
           out.handle(getLabel(), Joiner.on('/').join(values));
         }
       };