Merge "Document the tracing subscriber usage in README.md, better sample init" into main am: ca36a936b3

Original change: https://android-review.googlesource.com/c/platform/system/extras/+/2753571

Change-Id: I23a836d5a96ca4ee0ca8ae4974b6de733cae675c
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>