By default, if one log in a batch fails (ex: due to large size), the entire batch will be dropped. This PR sets the partial_success flag by default, so other logs will still be processed. We also expose partial_success as an optional argument to batch.commit(), so users can override the behavior if needed.

Note that most logger.log functions only typically write a single log entries, so this functionality is only strictly needed for batching. The only exception is when writing instrumentation logs, where a logger.log call may write two entries. For this reason, partial_success is always set to true for these calls

Fixes #448