CodeStream Integration Code-Level Metrics for Transactions

[nr-swilloughby]

This adds code-level metrics for transaction spans. (Future releases may add support for other trace types than transactions as well.)

Fixes Issue 493.

[iamemilio]

LGTM

[josephnaberhaus-wk]

@nr-swilloughby this was a major change. It breaks interface compatibility.

*Application used to fulfill this interface:

type example interface {
	StartTransaction(string) *newrelic.Transaction
}

but now it doesn't.

This caused some of our code to stop building once we upgraded to 3.18.0. We have fixed this issue on our end, but I wanted to give you a heads up.

[nr-swilloughby]

@josephnaberhaus-wk Thank you for the heads up. We'll push out a fix for this ASAP, most likely early next week.