Change fallback trace name to be more informative

[tiffon]

Fix #190.

[codecov]

Codecov Report

Merging #192 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #192      +/-   ##
=========================================
+ Coverage    91.4%   91.5%   +0.09%     
=========================================
  Files          95      95              
  Lines        2130    2130              
  Branches      434     434              
=========================================
+ Hits         1947    1949       +2     
+ Misses        162     160       -2     
  Partials       21      21

Impacted Files	Coverage Δ
src/constants/index.js	100% <100%> (ø)	⬆️
...neViewer/TimelineHeaderRow/TimelineViewingLayer.js	90.74% <0%> (+3.7%)	⬆️

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[black-adder]

Can we also add one of them scroll over info thingies (I'm a UX professional as you can tell) that says what root-span is missing means? ie It happens because the root service is poorly instrumented or the root span hasn't been persisted yet because it's the last span in a trace to be finished and persisted.

[tiffon]

@black-adder Added a ticket from your comment, #193.