Populate resource to OTLP proto data

[ThomsonTan]

Fixes #757

Changes

Populate resource to OTLP proto data and also update the readme for OTLP receiver with easier instructions to run and test it end-to-end with docker.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #758 (bb95dd1) into main (3bfbc93) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #758      +/-   ##
==========================================
+ Coverage   95.94%   95.95%   +0.01%     
==========================================
  Files         174      174              
  Lines        7127     7127              
==========================================
+ Hits         6838     6839       +1     
+ Misses        289      288       -1     

Impacted Files	Coverage Δ
...include/opentelemetry/sdk/common/circular_buffer.h	100.00% <0.00%> (+2.12%)	⬆️

[lalitb]

nit - Just wondering if line 35 - 40 can be replaced with single line:

resource_span->mutable_resource() = rec->ProtoResource();

using older implementation of ProtoResource.

[ThomsonTan]

I chose set_allocated_resource which could avoid allocate the Resource twice or more, but yes, it look simpler with mutable_resource. Updated the PR to the older implementation of ProtoResource as this optimization may not deserve the extra complexity.

[lalitb]

Thanks for adding the support. Looks good, perhaps we can add unit tests for the resources too.

[ThomsonTan]

Thanks for adding the support. Looks good, perhaps we can add unit tests for the resources too.

Added unit resource test to OTLP recordable test case. We may need more tests if we'd like to validate the OTLP exported result.