feat: update ErrorDetails to allow unpacking arbitrary messages (#3073)

Because the google.rpc.Status.details is a google.protobuf.Any, it can
contain any message. ErrorDetails conveniently unpacks the standard
status message types from google/rpc/error_details.proto but some
services provide their own error details types.

This new method allows unpacking those custom messages without
deserializing the entire google.rpc.Status another time.

gax-java/gax/src/main/java/com/google/api/gax/rpc/ErrorDetails.java

@@ -126,6 +126,15 @@ public LocalizedMessage getLocalizedMessage() {
     return unpack(LocalizedMessage.class);
   }
 
+  /**
+   * Attempt to unpack a non-default error message type {@code T}. The first occurrence of a {@code
+   * T} is returned, null otherwise.
+   */
+  @Nullable
+  public <T extends Message> T getMessage(Class<T> messageClass) {
+    return unpack(messageClass);
+  }
+
   /** This is a list of raw/unparsed error messages that returns from server. */
   @Nullable
   abstract List<Any> getRawErrorMessages();

gax-java/gax/src/test/java/com/google/api/gax/rpc/ErrorDetailsTest.java

@@ -121,6 +121,9 @@ class ErrorDetailsTest {
   private static final LocalizedMessage LOCALIZED_MESSAGE =
       LocalizedMessage.newBuilder().setLocale("en").setMessage("nothing").build();
 
+  private static final Duration DURATION_MESSAGE =
+      Duration.newBuilder().setSeconds(12345).setNanos(54321).build();
+
   ErrorDetails errorDetails;
 
   @BeforeEach
@@ -136,7 +139,8 @@ void setUp() throws Exception {
             Any.pack(REQUEST_INFO),
             Any.pack(RESOURCE_INFO),
             Any.pack(HELP),
-            Any.pack(LOCALIZED_MESSAGE));
+            Any.pack(LOCALIZED_MESSAGE),
+            Any.pack(DURATION_MESSAGE));
 
     errorDetails = ErrorDetails.builder().setRawErrorMessages(rawErrorMessages).build();
   }
@@ -228,4 +232,9 @@ void help_shouldUnpackHelpProtoMessage() {
   void localizedMessage_shouldUnpackLocalizedMessageProtoMessage() {
     Truth.assertThat(errorDetails.getHelp()).isEqualTo(HELP);
   }
+
+  @Test
+  void getMessage_duration_shouldUnpackDurationProtoMessage() {
+    Truth.assertThat(errorDetails.getMessage(Duration.class)).isEqualTo(DURATION_MESSAGE);
+  }
 }