OpenTelemetry template: use the interface name without the package as the default span name prefix

[mlaflamm]

This PR take care of #101.

Take for example this interface :

package mypackage

type MyInterface interface {
	MyMethod()
}

When the interface and the generated file are in the same package, the span names start with the interface name like this MyInterface.MyMethod. This PR doesn't change that.

When the interface and the generated file are NOT in the same package, the span name had an additional prefix that include the interface package. It used to be mypackage.MyInterface.MyMethod but after #92 it became _sourceMypackage.MyInterface.MyMethod.

With this PR, the default span name is always the same whether the interface and the generated file are in the same package or not i.e. MyInterface.MyMethod.

This is also possible to change default span name prefix with the SpanNamePrefix variable. For SpanNamePrefix=NewInterface, the generated span name would be NewInterface.MyMethod. If you want to include the package name, include it in the variable: SpanNamePrefix="mypackage.MyInterface".

[mlaflamm]

I forgot to mention that I also added the span status on error.

[hexdigest]

Thanks for the fixes @mlaflamm!