Proxy to send zipkin b3 headers

[samutamm]

Fix #578

• opentracing is deprecated (in favor of open-telemetry) so remove it and use "native" zipkin for tracing
• call zipkin-go SDK and remove explicit creation of spans and span.Finish()
• When tracing object is configured in HTTPServer, share the created tracer instance to Proxy filter via HTTPContext.
• Use zipkin to inject B3 headers if not present already

[codecov-commenter]

Codecov Report

Merging #579 (a844049) into main (cdc0780) will increase coverage by 0.01%.
The diff coverage is 97.22%.

@@            Coverage Diff             @@
##             main     #579      +/-   ##
==========================================
+ Coverage   80.10%   80.12%   +0.01%     
==========================================
  Files          95       95              
  Lines       10940    10956      +16     
==========================================
+ Hits         8764     8779      +15     
- Misses       1700     1701       +1     
  Partials      476      476              

Impacted Files	Coverage Δ
pkg/object/httpserver/mux.go	71.70% <ø> (-0.16%)	⬇️
pkg/filter/proxy/proxy.go	82.40% <96.55%> (+2.30%)	⬆️
pkg/filter/proxy/pool.go	79.78% <100.00%> (-0.22%)	⬇️
pkg/object/httpserver/spec.go	73.33% <0.00%> (-4.00%)	⬇️
pkg/object/mqttproxy/client.go	79.63% <0.00%> (-0.91%)	⬇️
pkg/filter/proxy/request.go	100.00% <0.00%> (+4.54%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cdc0780...a844049. Read the comment docs.