tbs: Add missing root transaction reporting

Spin-off from https://github.com/elastic/apm-server/issues/18084.

> Expose a metric which observes sampling decisions The idea is to surface a metric that can show a user scenarios when traces are missing a root transaction. This can be a counter which tracks each time a transaction group is sampled. Or a metric which can track valid unsampled traces which have a root transaction.

See https://github.com/elastic/apm-server/issues/18084#issuecomment-4205965959 for more details.

Determine whether this is worth doing.