outbound: Add a request_duration histogram for route backends #271

Job	Run time
meta	29s
rust	3m 55s
info	1s
rust-crates (linkerd-app-outbound)	1m 15s
actions	0s
linkerd-install	5m 9s
rust-crates (linkerd-http-prom)	34s
ship-it	4s
	11m 27s

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