Add DkmEntryBuilder to private metrics client API

Adds DkmEntryBuilder for use in private metrics client API. This class
is intended to be the interface for DKM metric collection. See design
document: go/pmc-dkm.

Bug: 433730638
Change-Id: Ieac9ec863ef574d9ad633579a496f1a91dd4c932
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6931070
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Jay Zhou <zhouzj@google.com>
Reviewed-by: Matt Dembski <dembski@google.com>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1515154}
NOKEYCHECK=True
GitOrigin-RevId: 6e30fd97f659f8cbe845b087157bede7521ee8ea