[Feature] Bring back transactionDurationThreshold for managed transactions

[llibdude]

By digging into old PRs I understand that the ability to configure managed transactions was not something desirable due to the fact the library should have full control of the transaction.

I see it was removed in this PR back in 2020 (related issue).

Request
Allow configuration of transactionDurationThreshold for managed transactions.

Reason
Under advanced topics in the documentation, it suggests we can create our own custom managed transactions so we can have the ability to control the transaction while still getting much of the instrumentation collected for free. The caveat was that we needed to add a custom blocking span. This is working great for me, however I noticed that the transaction no matter what still ends after 1 minute. Not only does it end, but it gets discarded.

I wonder if I am using Kibana RUM in a way not intended, but would be nice to have longer than 1 minute transactions that observes a longer duration workflow (such as a checkout funnel, etc) while still automatically tracking things like https requests so that I don't need to worry about it.

If not, could the documentation around custom transactions be updated to include this limitation?

Thanks for considering.