out_stackdriver: test_log_entry_format will output log entries to stdout

[avilevy18]

Adding new config boolean parameter "test_log_entry_format", if true, all requests sent to Google Cloud Logging will be printed to standard output instead.

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Testing
Before we can approve your change; please submit the following in a comment:

• Example configuration file for the change

[OUTPUT]
  Name        stackdriver
  Match       …
  test_log_entry_format True

• Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found
  valgrind --leak-check=full ./fluent-bit --conf=<path>

==3512885== 
==3512885== HEAP SUMMARY:
==3512885==     in use at exit: 0 bytes in 0 blocks
==3512885==   total heap usage: 8,287 allocs, 8,287 frees, 1,928,300 bytes allocated
==3512885== 
==3512885== All heap blocks were freed -- no leaks are possible
==3512885== 
==3512885== For lists of detected and suppressed errors, rerun with: -s
==3512885== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• Run local packaging test showing all targets (including any new ones) build.
• Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• Documentation required for this feature

Backporting

• Backport to latest stable release.

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[igorpeshansky]

Some preliminary comments…

[igorpeshansky]

LGTM