Opentelemetry-go: Update sampling decision names for spec update

[ThomsonTan]

OTel trace SDK spec was update with new names for sampling decision.

• A sampling Decision. One of the following enum values:
  
  
  
  • DROP - IsRecording() == false, span will not be recorded and all events and attributes
    
    will be dropped.
  
  
  • RECORD_ONLY - IsRecording() == true, but Sampled flag MUST NOT be set.
  
  
  • RECORD_AND_SAMPLE - IsRecording() == true AND Sampled flag` MUST be set.
  
  

[arminru]

There was a follow-up renaming it once again:
https://github.com/open-telemetry/opentelemetry-specification/pull/956/files

[MrAlias]

@arminru do you expect this to be "stable" at this point?

[arminru]

After consulting my 🔮 I'd say yes, feel free to go ahead!