Add OpenTracing Tracer resolving through ServiceLoader

[eyalkoren]

What does this PR do?

Adds the ability to automatically configure usage of Elastic's OpenTracing bridge in systems using ServiceLoader. An example for such system is tracer-resolver, which is used by various OpenTracing libraries, for example the Apache Camel OpenTracing component.

Checklist

• This is an enhancement of existing features, or a new feature in existing plugins
  • I have updated CHANGELOG.asciidoc
  • I have added tests that prove my fix is effective or that my feature works
  • I have made corresponding changes to the documentation

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💚 Build Succeeded

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Build stats

• Build Cause: Started by user eyalkoren

• Start Time: 2021-03-22T12:39:04.523+0000

• Duration: 52 min 0 sec

• Commit: dd34466

Test stats 🧪

Test	Results
Failed	0
Passed	1901
Skipped	15
Total	1916

Trends 🧪

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	1901
Skipped	15
Total	1916

[codecov-io]

Codecov Report

Merging #1708 (dd34466) into master (789406b) will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1708      +/-   ##
============================================
+ Coverage     58.32%   58.33%   +0.01%     
- Complexity      109     2383    +2274     
============================================
  Files           430      430              
  Lines         18899    18899              
  Branches       2617     2618       +1     
============================================
+ Hits          11022    11025       +3     
  Misses         7102     7102              
+ Partials        775      772       -3     

Impacted Files	Coverage Δ	Complexity Δ
...tic/apm/agent/report/ssl/TLSFallbackSSLSocket.java	39.13% <0.00%> (-2.18%)	20.00% <0.00%> (+20.00%)	⬇️
...o/elastic/apm/agent/profiler/SamplingProfiler.java	73.04% <0.00%> (+0.23%)	0.00% <0.00%> (ø%)
...ic/apm/agent/profiler/collections/LongHashSet.java	18.25% <0.00%> (+0.39%)	0.00% <0.00%> (ø%)
.../bci/bytebuddy/SoftlyReferencingTypePoolCache.java	93.75% <0.00%> (+12.50%)	9.00% <0.00%> (+9.00%)

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