fix(tracing): Make sure a valid source is set on a transaction event

[tonyo]

Transaction source should be one of the predefined values as described by the envelopes protocol: https://develop.sentry.dev/sdk/event-payloads/transaction/#transaction-annotations

The safest way to do for Go seems to be checking the source in Span.toEvent(): we don't currently have setters for Span.Name and Span.Source, so adjusting the source earlier may be not enough.

Fixes #588
Related to #511

[tonyo]

Now I'm looking at this again, lemme think a bit more about making Span.Source private 🤔

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.04 🎉

Comparison is base (5d969d0) 79.60% compared to head (328ce62) 79.64%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #637      +/-   ##
==========================================
+ Coverage   79.60%   79.64%   +0.04%     
==========================================
  Files          38       38              
  Lines        3888     3896       +8     
==========================================
+ Hits         3095     3103       +8     
  Misses        683      683              
  Partials      110      110              

Impacted Files	Coverage Δ
tracing.go	89.43% <100.00%> (+0.17%)	⬆️

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[tonyo]

Decided not to touch Span.Source for now: we should probably couple that with overall improvements around settings/getting transaction Source and Name.