Set a continuous reporting spec for MetricsReporterTest#SampleRate* tests am: 9b79e11288

Original change: https://android-review.googlesource.com/c/platform/art/+/2200424

Change-Id: Ia0cfb99f8e89732d602c923b9eae3b8f56187f26
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>