This PR follows up on the discussion in #2603 to add options for ddtrace logs to be written to a file. This is useful when troubleshooting.

Environment Variables

This PR introduces three new environment variables:

1. DD_TRACE_LOG_FILE - when used with debug mode, this forwards ddtrace logs to a specific file.
2. DD_TRACE_LOG_FILE_SIZE_BYTES - when used, sets the max file size. The default is 15 mb.
3. DD_TRACE_LOG_FILE_LEVEL used to control the log level defined for the RotatingFileHandler. It will accept values such as DEBUG, WARNING, ERROR, etc based on the logging library.

How it works

1. DD_TRACE_DEBUG controls the log level of the ddtrace logger, while DD_TRACE_LOG_FILE_LEVEL controls the log level of the RotatingFileHandler used with file creation. DD_TRACE_LOG_FILE_LEVEL defaults to debug mode, but logs aren't written unless DD_TRACE_LOG_FILE is configured.
2. Child loggers under the ddtrace follow the configured used by ddtrace.
   * Exception: The ddtrace-run logs under commands/ddtrace-run do not use DD_TRACE_LOG_FILE if DD_TRACE_DEBUG is enabled.
   * Example: Logs that start with program executable
3. Behavior is unaffected by the import order of ddtrace is imported relative to logging when using patch_all/patch.
4. The logging levels follow the logging module from the standard Python library.
5. Python 2 only: If the application is using DD_TRACE_DEBUG=true, a basic logging handler will need to be configured,
   ie: logging.basicConfig(). Otherwise, console logs will not be emitted due to No handlers could be found for logger "ddtrace". This logging behavior does not occur in Python 3.

Example usage

To get debug level logs written to a file:

1. DD_TRACE_DEBUG=true DD_TRACE_LOG_FILE=/path/to/my/debuglogs
2. DD_TRACE_LOG_FILE_LEVEL=DEBUG DD_TRACE_LOG_FILE=/path/to/my/debuglogs

Summary of Options

Further customization of the logging configuration is possible:

Checklist

• Added to the correct milestone.
• Tests provided or description of manual testing performed is included in the code or PR.
• Library documentation is updated.
• Corp site documentation is updated (link to the PR).