feat(storage): allow user to delete custom metadata to match underlying SDKs behavior

packages/firebase_storage/firebase_storage_platform_interface/lib/src/settable_metadata.dart

@@ -41,7 +41,7 @@ class SettableMetadata {
   final String? contentType;
 
   /// Additional user-defined custom metadata.
-  final Map<String, String>? customMetadata;
+  final Map<String, String?>? customMetadata;
 
   /// Returns the settable metadata as a [Map].
   Map<String, dynamic> asMap() {

packages/firebase_storage/firebase_storage_web/lib/src/utils/metadata.dart

@@ -48,13 +48,26 @@ storage_interop.SettableMetadata settableMetadataToFbSettableMetadata(
 storage_interop.UploadMetadata settableMetadataToFbUploadMetadata(
     SettableMetadata metadata,
     {String? md5Hash}) {
+  // We are removing null values from the customMetadata map.
+  // since we are setting the whole map, whereas in Android and iOS we can
+  // set individual keys.
+  final listOfCustomMetadata = metadata.customMetadata?.entries
+      .where((entry) => entry.value != null)
+      .map((entry) => MapEntry(entry.key, entry.value))
+      .cast<MapEntry<String, String>>();
+
+  final Map<String, String>? filteredCustomMetadata =
+      listOfCustomMetadata != null
+          ? Map.fromEntries(listOfCustomMetadata)
+          : null;
+
   return storage_interop.UploadMetadata(
     cacheControl: metadata.cacheControl,
     contentDisposition: metadata.contentDisposition,
     contentEncoding: metadata.contentEncoding,
     contentLanguage: metadata.contentLanguage,
     contentType: metadata.contentType,
-    customMetadata: metadata.customMetadata,
+    customMetadata: filteredCustomMetadata,
     md5Hash: md5Hash,
   );
 }

packages/firebase_storage/firebase_storage_web/lib/src/utils/metadata_cache.dart

@@ -31,7 +31,7 @@ class SettableMetadataCache {
       return _cache;
     }
 
-    final newMetadata = <String, String>{
+    final newMetadata = <String, String?>{
       ...?incoming.customMetadata,
       ...?_cache.customMetadata,
     };

tests/integration_test/firebase_storage/reference_e2e.dart

@@ -391,6 +391,27 @@ void setupReferenceTests() {
         expect(fullMetadata.customMetadata!['foo'], 'bar');
       });
 
+      test('delete metadata', () async {
+        Reference ref = storage.ref('flutter-tests').child('flt-ok.txt');
+        FullMetadata fullMetadata = await ref
+            .updateMetadata(SettableMetadata(customMetadata: {'foo': 'bar'}));
+        expect(fullMetadata.customMetadata!['foo'], 'bar');
+
+        fullMetadata = await ref.updateMetadata(
+          SettableMetadata(
+            customMetadata: {
+              'foo': null,
+            },
+          ),
+        );
+        expect(fullMetadata.customMetadata!['foo'], isNull);
+
+        // Setting it again
+        fullMetadata = await ref
+            .updateMetadata(SettableMetadata(customMetadata: {'foo': 'bar2'}));
+        expect(fullMetadata.customMetadata!['foo'], 'bar2');
+      });
+
       test('errors if property does not exist', () async {
         Reference ref = storage.ref('flutter-tests/iDoNotExist.jpeg');