fix: include missing proto files in npm distribution

[blumamir]

Which problem is this PR solving?

#1538
#1524
proto files from build directory of @opentelemetry/exporter-collector-proto and @opentelemetry/exporter-collector-grpc are not published to npm in v0.11.0, causing error message about missing files to be logged when package is used, and exporter to not work.

Short description of the changes

Added the protos folder to the "files" section of package.json.
Validated the fix with npm publish --dry-run:

npm notice 304B   build/protos/opentelemetry/proto/collector/README.md                       
npm notice 2.9kB  build/protos/opentelemetry/proto/common/v1/common.proto                    
npm notice 1.9kB  build/protos/opentelemetry/proto/collector/metrics/v1/metrics_service.proto
npm notice 19.2kB build/protos/opentelemetry/proto/metrics/v1/metrics.proto                  
npm notice 1.3kB  build/protos/opentelemetry/proto/resource/v1/resource.proto                
npm notice 2.6kB  build/protos/opentelemetry/proto/trace/v1/trace_config.proto               
npm notice 2.0kB  build/protos/opentelemetry/proto/collector/trace/v1/trace_service.proto    
npm notice 11.1kB build/protos/opentelemetry/proto/trace/v1/trace.proto      

those files where missing before the change, and appear after the fix is applied.

[obecny]

lgtm, thx for spotting that

[codecov]

Codecov Report

Merging #1539 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1539   +/-   ##
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  Coverage   93.82%   93.82%           
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  Files         154      154           
  Lines        4762     4762           
  Branches      951      951           
=======================================
  Hits         4468     4468           
  Misses        294      294