tools/metrics: Implement codegen for DKM

Domain Keyed Metrics are a type of Private Metrics that are keyed by
eTLD+1 and processed on Trusted Execution Environments.

This patch adds code generation for builders for DKM. Metrics are
defined in dkm.xml and builders are generated in
gen/components/metrics/private_metrics.

Fixed: 433728659
Change-Id: I42af6068c769c2e8f6b7affbb858c7507cae4f2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7016490
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Kamil Jarosz <kjarosz@google.com>
Reviewed-by: Matt Dembski <dembski@google.com>
Reviewed-by: Jay Zhou <zhouzj@google.com>
Cr-Commit-Position: refs/heads/main@{#1535878}
NOKEYCHECK=True
GitOrigin-RevId: cd58a92f7b51ad660707340f519dccab269f25db