Both init_tracing_subscriber() and init_otlp() were calling try_init()
on the global tracing dispatcher, causing "global default trace dispatcher
has already been set" at runtime when APP_OTEL_ENABLED=true.
Fix: simplify the API so init_tracing_subscriber() never installs the
subscriber — it either calls try_init() immediately (non-OTLP mode) or
returns without installing (OTLP mode, defer=true). init_otlp() now
builds the complete subscriber stack (registry + env_filter + fmt_layer +
otel_layer) and calls try_init() once.
init_tracing_subscriber() signature: (level, defer) → ()
init_otlp() signature: (endpoint, service_name, _, log_level) → Result
The fmt layer is replicated inside init_otlp() for the OTLP path.
8defac98ad