revise the reader semantics

checkpoint modifies the underlying log rather than reading timestamps.
reset replays without going back to the underlying log.
add a test

Bug: 32982362
Test: runtest --path frameworks/base/core/tests/coretests/src/android/metrics
Change-Id: I381b203a1c24fcd098d7df4d9d0a50bd8aaa1309
diff --git a/proto/src/metrics_constants.proto b/proto/src/metrics_constants.proto
index 8a3b9af..f06c8dcf 100644
--- a/proto/src/metrics_constants.proto
+++ b/proto/src/metrics_constants.proto
@@ -3777,6 +3777,9 @@
     // Package: Package of app that was autofilled
     AUTOFILL_SESSION_FINISHED = 919;
 
+    // meta-event: a reader has checkpointed the log here.
+    METRICS_CHECKPOINT = 920;
+
     // ---- End O Constants, all O constants go above this line ----
 
     // Add new aosp constants above this line.