Secret-safe telemetry emitted by IrohBeam.

Events use the [:iroh_beam, ...] prefix. Every span emits :start and :stop; exceptions additionally emit :exception. Measurements use native units (:duration in monotonic time and :bytes as a non-negative integer). Metadata is bounded to operation/profile/path/outcome/category values. Private keys, relay tokens, payloads, close reasons, socket addresses, endpoint IDs, tickets, and native debug chains are never included.