Implement record of tagged span data in tracing integration

[DoumanAsh]

This is prototype to attach tagged data for tracing spans
Please let me know if it is suitable and I'll see about unit tests if necessary

Fixes #653

[nrxus]

What's the reason behind putting these methods in TransactionData instead of directly in Transaction, Span, and TransactionOrSpan? This way it looks like TransactionData's intent was as an iterator of transaction data not as a way to add more data into it (especially since there is a set_data already on the other three structs I mentioned).

Additionally, it looks like this is a change in behavior? Previously data would be added to the Span directly if we were in an inner span (nor the root transaction) but now data will be added to the transaction directly?

[DoumanAsh]

What's the reason behind putting these methods in TransactionData

Because acquiring lock every time you need to add data point is pointless

Additionally, it looks like this is a change in behavior?

There is no change in behavior, data is added in the same places as before

@Swatinem Is this looking good then do you need unit tests? I looked over tracing layer and there is zero to none existing tests

[DoumanAsh]

Is this crate still maintained?

[Swatinem]

We often "abused" doctests to act as unit tests for a lot of things. Even though it would be good to have tests for this, I can also merge this without to speed things up here a bit.

[DoumanAsh]

Is there any utility to create some sort of dummy/test setup for sentry in order to create transactions and read its data later?
I'm wondering if there are some tests exists that check transaction creation via global hub?

[Swatinem]

There is a test utility that will create a new sentry with a testing transport that captures all the envelopes/events happening within a scope. You should be able to use that.

[DoumanAsh]

Created PR with basic test as follow up #703