feat(server): Track a spans/count_per_root_project metric

[jan-auer]

Adds a new metric c:spans/count_per_root_project@none that tracks the number of spans per root project with a transaction and decision tag. This will be used by span-based dynamic sampling for rebalancing.

The new metric is tracked in the two places where span metrics are emitted:

1. Event metric extraction
2. Span processing

Requires https://github.com/getsentry/sentry/pull/78992 Closes https://github.com/getsentry/projects/issues/198