[milo] Add metrics for gitiles latency
[milo] Add metrics for gitiles latency

Change-Id: I32f24f96683694a6f64a3a34446f961bcea2c938
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1506027
Commit-Queue: Ryan Tseng <2190fe9a39b4920ec7bd59c848ec5cccd163056f@chromium.org>
Reviewed-by: Dirk Pranke <6693ee08b084bb2ffa767f664c99b689395e0e4b@chromium.org>

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