refactor(opentracing): add tests and pass correct span contexts

[vigneshshanmugam]

• Fix the issue when the parent span(handle-request) was getting finished before the child span(fetch-fragment) is finished.
• Add tests for the opentracing instrumentation using MockTracer

[vigneshshanmugam]

we know this is a fragment span from operation name so removed the fragment

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@88c5850). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #236   +/-   ##
=========================================
  Coverage          ?   99.06%           
=========================================
  Files             ?       16           
  Lines             ?      642           
  Branches          ?      117           
=========================================
  Hits              ?      636           
  Misses            ?        6           
  Partials          ?        0

Impacted Files	Coverage Δ
lib/request-handler.js	99% <100%> (ø)
lib/fragment.js	100% <100%> (ø)

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[lmineiro]

👍

[vigneshshanmugam]

👍

[lmineiro]

👍