Rust

Add Envoy filter metadata example. #2285

Job	Run time
audit	9s
licenses	8s
bazel	1m 6s
examples (hello_world)	27s
msrv	1m 1s
examples (http_auth_random)	24s
nightly	28s
outdated	5m 5s
examples (http_body)	30s
examples (http_config)	28s
reactors (hello_world)	25s
stable	33s
examples (http_headers)	22s
reactors (http_auth_random)	22s
examples (grpc_auth_random)	24s
reactors (http_body)	25s
examples (envoy_filter_metadata)	21s
reactors (http_config)	24s
reactors (http_headers)	27s
reactors (grpc_auth_random)	21s
reactors (envoy_filter_metadata)	21s
	14m 11s

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