Lua bridge tracer

[ElvinEfendi]

What this PR does / why we need it:
I'm not sure yet but I think it might be valuable to include some Lua traces as well (this will particularly be useful when we have Lua plugin system). For now I'm thinking we can create spans for each Nginx Lua phase, that way one can see how long does it take for a phase to run for given request.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

[ElvinEfendi]

But there's no urgency to make this part of 0.23.0, maybe 0.24.0.
For now
/hold

[aledbf]

@ElvinEfendi how we use this? (I don't see a module for nginx)

[ElvinEfendi]

@aledbf it gets loaded dynamically as Lua module: https://github.com/opentracing/lua-bridge-tracer/blob/6354a7d7a179bcbb797957788b24f7267c6ccf39/CMakeLists.txt#L32

[codecov-io]

Codecov Report

Merging #3782 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3782   +/-   ##
=======================================
  Coverage   54.78%   54.78%           
=======================================
  Files          82       82           
  Lines        6120     6120           
=======================================
  Hits         3353     3353           
  Misses       2370     2370           
  Partials      397      397

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[ElvinEfendi]

/hold cancel

[ElvinEfendi]

After experimenting with this at #3783 I see no reason to not merge this now.

[aledbf]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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