Fix mongodb instrumentation not ending span correctly

[vmarchaud]

From my tests all the mongodb queries had the wrong duration so i inspected the code and found out that the span were never ended and automatically truncated when the root span is ended.

So i rewrote a little bit the patchs so they could correctly be ended and added the verification in the tests

cc @draffensperger @mayurkale22

[codecov-io]

Codecov Report

Merging #358 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #358      +/-   ##
=========================================
- Coverage   95.23%   95.2%   -0.04%     
=========================================
  Files         129     129              
  Lines        8592    8598       +6     
  Branches      634     638       +4     
=========================================
+ Hits         8183    8186       +3     
- Misses        409     412       +3

Impacted Files	Coverage Δ
src/stackdriver-monitoring.ts	81.05% <0%> (-2.11%)	⬇️
src/mongodb.ts	91.78% <0%> (-0.98%)	⬇️
test/test-mongodb.ts	93.79% <0%> (+0.08%)	⬆️

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[vmarchaud]

Test failures are related to Stackdriver so i believe we can ignore them for this PR

[mayurkale22]

Test failures are related to Stackdriver so i believe we can ignore them for this PR

I am working on fixing the build.

[mayurkale22]

Can you please rebase your branch with the master, hopefully it will fix build.

[vmarchaud]

I believe the PR can be merged now ?

[vmarchaud]

Ping @mayurkale22

[mayurkale22]

@vmarchaud Thanks for the contributions, PR looks good. If no other comments from @draffensperger I will merge this PR later today.

[draffensperger]

LGTM, thanks for the fixes and comments.