Go agent v3.6.0

3.6.0 New Features

Added support for adding custom attributes directly to spans. These attributes will be visible when looking at spans in the distributed tracing UI.
Example:
```
txn := newrelic.FromContext(r.Context())
sgmt := txn.StartSegment("segment1")
defer sgmt.End()
sgmt.AddAttribute("mySpanString", "hello")
sgmt.AddAttribute("mySpanInt", 123)
```
Custom attributes added to the transaction with txn.AddAttribute are now also added to the root span and will be visible when looking at the span in the distributed tracing UI. These custom attributes can be disabled from all destinations using Config.Attributes.Exclude or disabled from spans specifically using Config.SpanEvents.Attributes.Exclude.
Agent attributes added to the transaction are now also added to the root span event and will be visible when looking at the span in the distributed tracing UI. These attributes include the request.uri and the request.method along with all other attributes listed in the attributes section of our godocs. These agent attributes can be disabled from all destinations using Config.Attributes.Exclude or disabled from spans specifically using Config.SpanEvents.Attributes.Exclude.

Fixed an issue where it was impossible to exclude the attributes error.class and error.message from the root span. This issue has now been fixed. These attributes can now be excluded from all spans using Config.Attributes.Exclude or Config.SpanEvents.Attributes.Exclude.
Fixed an issue that caused Go's data race warnings to trigger in certain situations when using the newrelic.NewRoundTripper. There were no reports of actual data corruption, but now the warnings should be resolved. Thank you to @blixt for bringing this to our attention!