Added context injection and logger limit

[MarkSeufert]

This PR makes the following additions to the logging SDK:

• The SDK’s Logger injects the relevant fields of the LogRecord if the user didn’t specify them. The fields that get injected are timestamp, severity, traceid, spanid, and traceflags.
• Added a logger limit to the SDK LoggerProvider (relevant spec issue), where the GetLogger(name) method returns a noop logger if the limit has been reached.
• Unit tests for the above functionality.

The CI tests will fail until the recent API PR gets merged since it uses its trace_flag -> trace_flags change and Severity:kInvalid = 0 change.

cc - @xukaren @alolita

[alolita]

Does this work?

gtest_add_tests(
TARGET ${testname}
TEST_PREFIX logs.
TEST_LIST ${testname})