Correlate logs to spans with APM agents

When logs are correlated, not all agents capture the `span.id` when a span is currently active.

Related Kibana issue: https://github.com/elastic/kibana/issues/159861

### Proposal & steps
- [ ] make APM agents correlate logs on `span.id` in addition to `trace.id` and `transaction.id`.
- [ ] specification change
- [ ] implementation for most agents
- [ ] per-agent documentation update for logs correlation fields
- [ ] general documentation update for correlation fields.
- in order to align with https://github.com/elastic/apm-data/issues/82, always report `span.id` even for transactions would be more consistent. For existing agents apm-data will provide a fallback.
- for OpenTelemetry, the logs are already using `span.id`, thus no change would be required.