Add a WithDummyInMemory function - useful for tests that want a fake metric store
Add a WithDummyInMemory function - useful for tests that want a fake metric store

BUG=

Review URL: https://codereview.chromium.org/1823163003

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