Get OTel pipeline statistics (admin/system view).

Internal pipeline statistics for self-observability.

All fields are cumulative process-lifetime counters. Every one of them is also sampled every 60 seconds by the plugin's stats sampler and written to the unified metrics store as a delta counter named otel.<field> (SourceKind::Node, node_id 0) — see pipeline_stat_deltas in plugin.rs. Adding a field here means adding it there too, otherwise the new counter is only ever visible through the API response.