Add scope manager support

[mohabusama]

Resolve #54

Changes

Add support for tracer scope manager support introduced in opentracing==2.0.

I am not fully decided on the order of detecting parent spans. The proposed implementation is the following:

~1. Using opentracing.tracer.active_span managed by the tracer context manager. The new span will be using the scope manager.~ ~2. Using span_extractor.~ ~3. Detecting span in kwargs.~ ~4. Using call stack frames inspection.~

1. Using span_extractor.
2. Detecting span in kwargs.
3. Using opentracing.tracer.active_span managed by the tracer context manager. The new span will be using the scope manager.
4. Using call stack frames inspection.

as span_extractor and passing spans in kwargs is more explicit.

Please let me know what you think.

Release strategy

We don't have to release a new version right away after merging. We are planning to first test the changes in our production systems, fix any related bugs observed and confirm that no breaking changes are introduced.

[codecov[bot]]

Codecov Report

Merging #55 (266b805) into master (e681bdd) will increase coverage by 0.13%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
+ Coverage   98.68%   98.81%   +0.13%     
==========================================
  Files           9        9              
  Lines         379      423      +44     
==========================================
+ Hits          374      418      +44     
  Misses          5        5              

Impacted Files	Coverage Δ
opentracing_utils/decorators.py	100.00% <100.00%> (ø)
opentracing_utils/libs/_django.py	97.43% <100.00%> (+0.21%)	:arrow_up:
opentracing_utils/libs/_flask.py	97.33% <100.00%> (+0.19%)	:arrow_up:
opentracing_utils/libs/_requests.py	100.00% <100.00%> (ø)
opentracing_utils/libs/_sqlalchemy.py	100.00% <100.00%> (ø)
opentracing_utils/span.py	100.00% <100.00%> (ø)

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[mohabusama]

@vetinari @pitr @dneuhaeuser-zalando Please let me know what you think.

[dneuhaeuser-zalando]

It could be useful if we change to the following order: [...] as span_extractor and passing spans in kwargs is more explicit.

Agreed. Using span_extractor or passing a span clearly and explicitly communicates what the user wishes to happen in that specific location. I think that should override a scope or at the very least lead to a warning.

[mohabusama]

@mvalkon you might be interested in this one.

[mohabusama]

@vetinari @pitr this one is ready, we have tested it in production.

[mohabusama]

👍

[pitr]

👍