feat: adding docs for payment metadata (#124)

Co-authored-by: vacwmX <vacwm01@gmail.com>
Co-authored-by: ok300 <106775972+ok300@users.noreply.github.com>

snippets/dart_snippets/lib/metadata.dart

@@ -0,0 +1,51 @@
+import 'package:breez_sdk/breez_sdk.dart';
+import 'package:breez_sdk/bridge_generated.dart';
+
+Future<void> setPaymentMetadata({required String paymentHash, required String metadata}) async {
+  // ANCHOR: set-payment-metadata
+  await BreezSDK().setPaymentMetadata(hash: "target-payment-hash", metadata: '{"myCustomValue":true}');
+  // ANCHOR_END: set-payment-metadata
+}
+
+Future<void> filterPaymentMetadata() async {
+  // ANCHOR: filter-payment-metadata
+  List<MetadataFilter> metadataFilters = [
+    MetadataFilter(
+      jsonPath: "myCustomValue",
+      jsonValue: "true",
+    ),
+  ];
+
+  await BreezSDK().listPayments(
+    req: ListPaymentsRequest(
+      metadataFilters: metadataFilters
+    ));
+  // ANCHOR_END: filter-payment-metadata
+
+  // ANCHOR: filter-payment-metadata-string
+  metadataFilters = [
+    MetadataFilter(
+      jsonPath: "customerName",
+      jsonValue: '"Satoshi Nakamoto"',
+    ),
+  ];
+  // ANCHOR_END: filter-payment-metadata-string
+
+  // ANCHOR: filter-payment-metadata-object
+  // This will *NOT* work
+  metadataFilters = [
+    MetadataFilter(
+      jsonPath: "parent.nestedArray",
+      jsonValue: "[1, 2, 3]",
+    ),
+  ];
+
+  // Any of these will work
+  metadataFilters = [
+    MetadataFilter(
+      jsonPath: "parent.nestedArray",
+      jsonValue: "[1,2,3]",
+    ),
+  ];
+  // ANCHOR_END: filter-payment-metadata-object
+}