[infra] Enable configuring alerts for builders

This updates the builder declarations to output a custom metric that can
be used in our alerting and monitoring configs to determine what builder
alerting is enabled for. This enables controlling whether a builder
alerts by changing the value of the alerts_enabled parameter. The
default behavior is that alerts are enabled for builders that have
gardener rotations set or that are automatically triggered by the CQ.

Bug: 400765628
Change-Id: I3f1e69433cb2c4fc98157a05a2188a9a3f144f95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6863193
Commit-Queue: Garrett Beaty <gbeaty@google.com>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1512663}