Rust

Add Envoy filter metadata example. #2280

Job	Run time
audit	6s
bazel	1m 9s
examples (hello_world)	21s
examples (http_auth_random)	22s
licenses	11s
msrv	1m 0s
examples (http_body)	21s
examples (http_config)	24s
nightly	28s
examples (http_headers)	22s
outdated	3s
examples (grpc_auth_random)	23s
stable	31s
examples (envoy_filter_metadata)	1s
reactors (hello_world)	22s
reactors (http_auth_random)	19s
reactors (http_body)	19s
reactors (http_config)	20s
reactors (http_headers)	14s
reactors (grpc_auth_random)	19s
reactors (envoy_filter_metadata)	20s
	7m 55s

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