Done:

• Actual command-started events
• Simulate command-started events by upconverting legacy writes
• Report operation_id and request_id with command-started events
• Make an internal function mongoc_apm_command_started_init (&event, lots of params...).
• Refactor upconversion code with actual write-command code in _mongoc_write_command
• Remove $readPreference from command-started event (tests should now pass with 3.2 on all topologies)
• Simulate command-started events by upconverting legacy query and getmore (tests should now pass on all server version)
• Remove mongoc_cluster_run_command_rpc
• Test mongoc_apm_command_failed_get_error
• Command-succeed events (merge those parts of spec tests into our repo)
• Command-failed events (merge those parts of spec tests into our repo)
• Document all new functions and structs, note that callbacks must copy event data to modify it, or to keep it after the callback invocation
• Write an APM Guide in the docs
• Prohibit setting APM callbacks directly on a pooled client, only allow setting them on the pool (head-start on CDRIVER-938)
• Test that multi-batch inserts report different request_ids and the same operation_ids, in both legacy and write-command mode