doc: add example to list

Signed-off-by: Martijn Swaagman <[email protected]>

Author: Swaagie

README.md

@@ -22,6 +22,7 @@
 - [HTTP Response body](./examples/http_body/)
 - [HTTP Configuration](./examples/http_config/)
 - [gRPC Auth (random)](./examples/grpc_auth_random/)
+- [Filter metadata](./examples/filter_metadata/)
 
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