Add histograms for ML Service pre-request metrics

Add suffix-based histograms for the following metrics added in Chrome OS CL
crrev.com/c/1312494.
* Numerical histograms for elapsed time, CPU time and RAM usage of
  LoadModel, CreateGraphExecutor and Execute
* Enum histogram for LoadModel, CreateGraphExecutor and Execute events

Bug: 892999
Change-Id: Idba5227aaec328e0f13fbc9116d88f47dc72a452
Reviewed-on: https://chromium-review.googlesource.com/c/1370150
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Andrew Moylan <amoylan@chromium.org>
Commit-Queue: Xinglong Luan <alanlxl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617026}
2 files changed