Add getAll to Getter for repeated metadata

It's known that some metadata can be repeated on a carrier. W3C baggage is one example of this in HTTP request headers. The current get API returns a String, and for performance reasons we may not want to check all headers to combine repeated headers when we don't expect there should be repeated headers.

This updates the one non-deprecated main code implementation of Getter (in ObservationGrpcServerInterceptor) to implement the new getAll method.

Author: shakuzen

micrometer-core/src/main/java/io/micrometer/core/instrument/binder/grpc/ObservationGrpcServerInterceptor.java

@@ -21,9 +21,11 @@
 import io.grpc.ServerCall.Listener;
 import io.micrometer.observation.Observation;
 import io.micrometer.observation.ObservationRegistry;
+import io.micrometer.observation.transport.Propagator;
 import org.jspecify.annotations.Nullable;
 
 import java.net.URI;
+import java.util.Collections;
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.function.Supplier;
@@ -61,10 +63,21 @@ public ObservationGrpcServerInterceptor(ObservationRegistry registry) {
     public <ReqT, RespT> Listener<ReqT> interceptCall(ServerCall<ReqT, RespT> call, Metadata headers,
             ServerCallHandler<ReqT, RespT> next) {
         Supplier<GrpcServerObservationContext> contextSupplier = () -> {
-            GrpcServerObservationContext context = new GrpcServerObservationContext((carrier, keyName) -> {
-                Metadata.Key<String> key = KEY_CACHE.computeIfAbsent(keyName,
-                        (k) -> Metadata.Key.of(keyName, Metadata.ASCII_STRING_MARSHALLER));
-                return carrier.get(key);
+            GrpcServerObservationContext context = new GrpcServerObservationContext(new Propagator.Getter<Metadata>() {
+                @Override
+                public @Nullable String get(Metadata carrier, String keyName) {
+                    Metadata.Key<String> key = KEY_CACHE.computeIfAbsent(keyName,
+                            (k) -> Metadata.Key.of(keyName, Metadata.ASCII_STRING_MARSHALLER));
+                    return carrier.get(key);
+                }
+
+                @Override
+                public Iterable<String> getAll(Metadata carrier, String keyName) {
+                    Metadata.Key<String> key = KEY_CACHE.computeIfAbsent(keyName,
+                            (k) -> Metadata.Key.of(keyName, Metadata.ASCII_STRING_MARSHALLER));
+                    Iterable<String> all = carrier.getAll(key);
+                    return all == null ? Collections.emptyList() : all;
+                }
             });
             context.setCarrier(headers);
 

micrometer-observation/src/main/java/io/micrometer/observation/transport/Propagator.java

@@ -17,6 +17,8 @@
 
 import org.jspecify.annotations.Nullable;
 
+import java.util.Collections;
+
 /**
  * Inspired by OpenZipkin Brave and OpenTelemetry. Most of the documentation is taken
  * directly from OpenTelemetry.
@@ -75,14 +77,40 @@ interface Getter<C> {
 
         /**
          * Returns the first value of the given propagation {@code key} or returns
-         * {@code null}.
+         * {@code null}. This method should be preferred over
+         * {@link #getAll(Object, String)}} for better performance in cases where it is
+         * expected that the key is not repeated.
          * @param carrier carrier of propagation fields, such as an http request.
          * @param key the key of the field.
          * @return the first value of the given propagation {@code key} or returns
          * {@code null}.
          */
         @Nullable String get(C carrier, String key);
 
+        /**
+         * Get all values of the given propagation {@code key}, if any exist. This should
+         * only be used when it is expected that the key may be repeated.
+         * {@link #get(Object, String)} should be preferred in other cases for
+         * performance.
+         * @param carrier carrier of propagation fields, such as an http request.
+         * @param key the key of the field.
+         * @return all values of the given propagation {@code key} or returns an empty
+         * {@code Iterable} if no values are found.
+         * @implNote For backward-compatibility, a default implementation is provided that
+         * returns a list with the value of {@link #get(Object, String)} or an empty list
+         * if no values are found. Implementors of this interface should override this
+         * method to provide an implementation that returns all present values of the
+         * given propagation {@code key}.
+         * @since 1.16.0
+         */
+        default Iterable<String> getAll(C carrier, String key) {
+            String firstValue = get(carrier, key);
+            if (firstValue == null) {
+                return Collections.emptyList();
+            }
+            return Collections.singletonList(firstValue);
+        }
+
     }
 
 }