Merge "Document the tracing subscriber usage in README.md, better sample init" into main am: ca36a936b3 am: a14b14c067

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

Change-Id: I3317e5d5d2d5a7524f809d59e03ebd1071131852
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>