improved template for 'go.elastic.co/apm/v2' traces

[bars43ru]

• fixed template elasticapm (call Send() in _d.captureError())
• added an option to regulate the use of SetLabel to register function parameters
• added option to set SpanType

[bars43ru]

@gvencadze this MR implements all the features that we discussed in https://github.com/hexdigest/gowrap/pull/57. I propose to continue further discussion here.

[bars43ru]

@hexdigest, thanks for the quick reaction to MR!