fix(tracing): use global provider by default, but allow custom providers

[Nomango]

Description (what this PR does / why we need it):

Tracing middleware uses global provider by default, but allow custom providers.

Which issue(s) this PR fixes (resolves / be part of):

fixes #2802

Other special notes for the reviewers:

No break changes.

[codecov-commenter]

Codecov Report

Merging #2803 (5b9be9d) into main (520b321) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

❗ Current head 5b9be9d differs from pull request most recent head c40886c. Consider uploading reports for the commit c40886c to get more accurate results

📣 This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

@@            Coverage Diff             @@
##             main    #2803      +/-   ##
==========================================
- Coverage   84.52%   84.47%   -0.06%     
==========================================
  Files          88       88              
  Lines        3923     3923              
==========================================
- Hits         3316     3314       -2     
- Misses        432      434       +2     
  Partials      175      175              

Impacted Files	Coverage Δ
middleware/tracing/tracing.go	92.30% <ø> (ø)
middleware/tracing/tracer.go	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.