Add Ostream log exporter and tests

[kxyr]

This PR implements an exporter to Ostream for the logging pipeline, as part of Issue # 337.

This PR adds:

• LogRecord change from a unique_ptr to a shared_ptr when passed from Logger SDK --> processor --> exporter, as proposed as a solution in # 412. This change applies to many test files, processor interfaces and implementations, and across the exporter interface and implementations.
• A OStream Log Exporter implementation. The OStreamLogExporter takes the batch of LogRecords from the processor, and sends each field to an OStream. The default configuration is to send the data to std::cout.
• Unit tests for the OStream exporter to cout, cerr, and clog, and an integration test for the full logging pipeline from Logger API --> LoggerSDK --> Simple Log Processor --> OStream Log Exporter

Note that all changes in log_record.h, logger.h, and noop.h in this PR can be ignored, as they are covered by # 422. However, including them here allows them to pass the CI and for this PR to be merged without # 422.

cc @MarkSeufert @alolita

[codecov-io]

Codecov Report

Merging #13 (644a625) into master (c5c82fd) will decrease coverage by 0.03%.
The diff coverage is 91.58%.

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
- Coverage   94.53%   94.49%   -0.04%     
==========================================
  Files         179      182       +3     
  Lines        7702     7888     +186     
==========================================
+ Hits         7281     7454     +173     
- Misses        421      434      +13     

Impacted Files	Coverage Δ
api/include/opentelemetry/logs/logger.h	100.00% <ø> (ø)
sdk/include/opentelemetry/sdk/logs/exporter.h	100.00% <ø> (ø)
sdk/include/opentelemetry/sdk/logs/processor.h	100.00% <ø> (ø)
...lude/opentelemetry/sdk/logs/simple_log_processor.h	100.00% <ø> (ø)
sdk/test/logs/logger_provider_sdk_test.cc	84.84% <0.00%> (ø)
sdk/test/logs/simple_log_processor_test.cc	96.22% <ø> (ø)
sdk/test/logs/logger_sdk_test.cc	84.61% <50.00%> (ø)
...ude/opentelemetry/exporters/ostream/log_exporter.h	64.70% <64.70%> (ø)
api/include/opentelemetry/logs/log_record.h	100.00% <100.00%> (ø)
exporters/ostream/src/log_exporter.cc	100.00% <100.00%> (ø)
... and 9 more

[alolita]

Lgtm mostly. Pl add inline comments esp for functions
e.g. in exporters/ostream/src/log_exporter.cc