OTel file exporter: option to stop writing metric records (file grows ~50-70 MB/day) #5344

With COPILOT_OTEL_FILE_EXPORTER_PATH set, the CLI writes metric records ({"type":"metric",...}) into the same JSONL file as spans. On one machine (CLI 1.0.103, Windows) the file reached 9.3 GB over ~6 months; 6,052,252 of 6,306,070 lines (96%) are metric records. Consumers that only need spans for token and cost accounting (for example TokenTracker) still have to read past them.

OTEL_METRICS_EXPORTER=none does not stop the metric records, and I could not find a documented setting that turns them off for the file exporter.

Request: a setting that disables metric export to the file (or writes metrics to a separate path), so a spans-only file stays small.

Related: #5005 (metric streams have no service.instance.id).