[server] Log X-Request-Id as a request log entry field.
[server] Log X-Request-Id as a request log entry field.

X-Request-Id is generated by Envoy when it sends a request to
an upstream. Emitting this header as a field would allow us to
correlate Envoy logs with ours.

R=ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org

Change-Id: I6400b6dc7cf4e342667a2428225781f1ba9c9009
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2120934
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go