feat(spans): extract transaction breakdowns into measurements

[DominikB2014]

closes #4598

More details found in the issue above. This PR extracts transaction breakdowns into measurements, so that they are automatically stored in the eap dataset. (Snuba does not look at breakdowns at all, but it automatically stores all measurements) We are moving the insights overview pages to eap, and the product relies on these breakdown values.

[iambriccardo]

LGTM, are we eventually planning to send measurements directly from the SDK instead of building them from breakdowns?

[DominikB2014]

are we eventually planning to send measurements directly from the SDK instead of building them from breakdowns
@iambriccardo Good question! I'm planning to to sync with the SDK team about that one, but i think that's what we should do eventually. Regardless, we'll likely have to keep this code for sometime to provide backwards compatibility

[iambriccardo]

are we eventually planning to send measurements directly from the SDK instead of building them from breakdowns

@iambriccardo Good question! I'm planning to to sync with the SDK team about that one, but i think that's what we should do eventually. Regardless, we'll likely have to keep this code for sometime to provide backwards compatibility

Thanks for the clarification! Keeping it for backwards compatibility sounds good to me.