Add metrics related to V4 database startup performance

To understand performance loss due to the delay from mmap'ing the SB
database, this CL adds two metrics:
- How long does it take to create the database?
- Are the stores ready immediately after startup, or do they need an
  update from the server?

Bug: b/330359965
Change-Id: Ief2ab0e26800a2fe2deec1d266fdbd3105edc68d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5540732
Reviewed-by: thefrog <thefrog@chromium.org>
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1301401}