Add span_extractor argument to requests tracing

[Birne94]

We want to use this package to send traces from our services to lightstep. Currently, tracing for the requests library does not support passing a custom span extractor, which I added as part of this PR.

[codecov[bot]]

Codecov Report

Merging #52 (13243a2) into master (9c89e1f) will decrease coverage by 1.08%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
- Coverage   98.37%   97.29%   -1.09%     
==========================================
  Files           9        9              
  Lines         370      370              
==========================================
- Hits          364      360       -4     
- Misses          6       10       +4     

Impacted Files	Coverage Δ
opentracing_utils/libs/_requests.py	100.00% <100.00%> (ø)
opentracing_utils/libs/_flask.py	97.05% <0.00%> (-2.95%)	:arrow_down:
opentracing_utils/libs/_django.py	97.22% <0.00%> (-2.78%)	:arrow_down:

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[Birne94]

I am not sure why the coverage for the flask and django integrations went down with this PR, I did not touch those.

[pitr]

👍

[Birne94]

👍

[Birne94]

@pitr Thanks for merging this! Is there a release planned for the near future or what is the release plan for this library?

[pitr]

plz +1 #53 to bump version

[Birne94]

plz +1 #53 to bump version

done!

[pitr]

0.21.0 should be now out