Transform Elastic APM tracing fields into ECS fields

[sethmlarson]

• Elastic APM logging fields: https://github.com/elastic/apm-agent-python/blob/a898c343bc075e2af5c60ac20ad422cc5290b877/elasticapm/handlers/logging.py#L229
• ECS Tracing Fields: https://www.elastic.co/guide/en/ecs/current/ecs-tracing.html
• span.id should be done as well, will be released in next ECS: APM/Log correlation: add span ID and trace ID ecs#99