Add `transaction.sampled` to errors

[simitt]

Quoting @sqren:

If an error occurs during an unsampled transaction, transaction.id will still be added to the error. When querying for errors it is therefore not possible to easily find one that happened during a sampled transaction.

Possible solutions

Add transaction.sampled to errors
This will make it trivial to filter for errors that happened during a sampled transaction.

[simitt]

@elastic/apm-agent-devs for current/old versions of the agents, this field will not be set on errors.

If not set, I will leave the transaction.sampled: null, and not default to true as we do for sending up transaction docs.

[makwarth]

Closed by #1664