Add `X-Envoy-Upstream-Service-Time` header (or equivalent) support on proxy

[lbroudoux]

The Envoy proxy always returns a X-Envoy-Upstream-Service-Time header in its responses to provide information on the time taken (in ms) to wait for the upstream/backend service. See the Envoy Doc for reference and exact definition. I think it can be a nice idea to add this header too or an equivalent X-Reshapr-Upstream-Service-Time