[v1][Metrics] Add design doc (vllm-project#12745)

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>
Co-authored-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>
Co-authored-by: Cody Yu <hao.yu.cody@gmail.com>

Author: 3 people